I know this situation because I'm exactly such one, and it was diagnosed by medical professionals.
Making a humor, or "avoiding" by neglecting; would not give any positive outcome if your friend is unable to understand unwritten social norms and what you "pretend" with your neglecting.
Tell him; straightly yet friendly; the professor and some people in the class is not liking it. Best if you can friendly explain, exactly where exactly why you/the professor are feeling it irritating. May be the professor forget what to tell when the question is coming. Or may be the friend's voice is loud, Or may be the professor have an ego problem (might be; a bitter truth. Everyone do have it). Avoid sign-languages and gestures to talk with her/him.
If a question seems very stupid or irrelevant to you; maybe the question is actually on-topic; and you/ your professor failed to realize why it was linked to topic. Or maybe you and your professor simply failed to go out of track. Unique-most questions look silly. Such as "Of an empty, white, coverless notebook; Which would be the front side and which one is back-side?" or for say "Why two light-beams cross each-other without any collision?" or "why roundish is used as most arbitrary shape?" or "is your red same as my?" or "why bicycle doesn't fall when the wheels rotate?"
In fact, Newton's question, why the apple fall to down; was funny enough, at least to me. Who doesn't know apple falls to down. Why he didn't thought to ate the apple. And it is that only-difference between our kettle and James Watt's kettle.
Or even not on-topic; the question do have some value to the asker's own.
You may suggest that friend to write the questions, but that would not solve the student's problem because soon he was going to ask about. That will cause a great misery to the student. Suggest him to develope some brief/symbolic/graphical notation so that he can quickly write it down. In fact. I had to and have to do this. There exist more modern option... Record the lecture with a smartphone/ electronic recorder, if the student can afford it and the institute permit it using in the class. Then the student could more calmly write down the question, and if he miss a portion of lecture, that portion could be later on found from the recorder.
It's gone what could we do about that student. Now what can we do about US? Is it the the student's fault? Actually it is our-fault. But what is a problem of mass, is considered as "normal". Only what is a problem for rare group of people (and sometimes not at all a problem); it could be "distinguished" (and as a problem, by norm).
In the above case (as told by OP); we the majority actually doing some mistake:
We're forgetting that everyone here in the institute, is for learning. It may apparently seems to "the student is interfering our learning". Actually it is opposite... we are learning less... only what the teacher is saying, and just passing-by the class for the day. But the autistic student in OP is simply want to learn much more than us, in his own way; and actually we're interfering that student's learning. (with providing a disrespectful and wrong (opposite) feedback).
We're getting 'irritated' so easily when someone doing a 'silly question'. That means we're not only intolerant to slightest disturb, we're unable to think about links between subjects or topics. In other hand, that student alone is tolerating the angry professor, the angry classmates, and attending the class everyday; just to learn.
If we the group-of-animal can't tolerate another group-of animal's inability to learn customs rituals and and mannerisms; it is plainly our fault. The other group of animal is constitutively different.
Many soft-teachers too sometimes doesn't allow students to interfere in lecture, there is a cause. If we start give the right to everyone to interfere the professor's talk; only that one autistic student would use it properly. Many other will really use to make disturb. If we allow bring electronic recorder to class; only that one autistic student would use the time to note the question. Many-others would start gossip or start practice inattentiveness. So, simply, even if the case is this; the problem is not of that 1 particular student. The problem is inside the mass.
So the one and only positive solution is, if we all could change ourselves instead changing or manipulating that one.
I know; this solution might not be possible to implement, but it is the bitter truth... only 'positive' solution.
There are some intermediate solutions ; where, we too can accept our fault as well as could request some compromise with the student.
"all the professors I have shared with this classmate (he and I are the same year in the same field of study) have followed the same pattern of humoring him to an extent by attempting to answer his questions, or by thanking him for his interjections, but eventually just ignoring him."
If the professor do not listen the questions by the students carefully; then certainly the professor is bad as teacher; maybe excellent as researcher. Research and teaching is not exactly same thing. But We can't blame for it to the professor- just because it is the biological-nature of that professor- just like the uniqueness of the student. We can tell this fact to the student. So we can request the student to study the behavior of the teacher,and behave accordingly, so that the student can continue "taking" from the teacher without going into conflict with the teacher. Especially in a research-institute, professors are usually employed for only the research-quality or research-talent. So very unfortunately, sometimes the teaching-psychology and sympathy get disconnected from academic-practice. But there is no reason for you to think what the professor is concluding, is ultimate. If the professor advocates for not to do questions, that is surely harmful.
For an autistic person; it is really difficult to understand/ predict some-other person's behaviour. So best if the student get some support about interpreting tonal-languages (talk-tones, pauses, speed etc) and body-language, etc, from a professional and good counsellor/ psychologist expert in autism. (in fact I would suggest this as the must-to do. Because this include clinical-professional advise). For him; than behave correctly; it is much more important to understand and predict the others' behaviour; at least to some extent.
No-one in this universe is more sillier or smarter than any other. Maybe, whom we all are thinking as silly; contains ability to flourish into wonderful one. It depends upon the rest of the world (including all of us) , that whether it will flourish, or ged destroyed with time.