I can imagine several scenarios, but don't have much information here to go on. The most hopeful one is that it is just a communication mismatch, or a mismatch of expectations. The only way to solve that, I think, is directly by asking him for guidance to the end. If
If he treats other students differently it could be a variety of things. Hopefully he respects you and your abilities, but if not you have a serious problem. Is it that the other students are more advanced already? Or that they have worked with him for a long time?
Another possibility is that he is overextended in his own work and advising and you are just the one with the short straw. Finding a different advisor might be a solution to that, if you can find someone more compatible who works in a close enough field that you aren't set back too much. I've mentioned in answers to other questions that some junior faculty are working toward tenure themselves and don't have time to advise effectively.
A wild, and unlikely, scenario, is that he is afraid of your abilities. That is rare, but possible. The correct behavior on his part, if he sees you as a superstar eclipsing his own, would be to get you going and get out of your way. This is the Good Will Hunting scenario.
Another answer here is also possible, though not all doctoral students have yet made the transition of thinking deeply enough to come up with their own problem. It is the ideal, to be sure, but takes seasoning and maturing that not all students attain until they finish their degrees (I'm an example, actually).
You also mention working in a well worn subfield in which problems are just hard to come up with and results even more so. When a field of maths is new there is little known so a lot can be learned quite quickly. As the field matures it gets harder and harder and mathematics moves on to something newer, though not all mathematicians do so. This was exemplified in the switch from Classical Real Analysis to Functional Analysis fifty or so years ago.
I don't expect that I gave you an answer here, other than to try to talk to your advisor and improve communications, but hope I gave something to think about at least. Maybe one of them will lead to a solution.