This is more an extended comment, but here goes.
My PhD advisor had me work on a topic (in applied probability) that he knew little about. I survived and got a PhD, but it was not pleasant.
He's seems to have thought that the area was much more interesting/useful/productive than it was, and had wildly unrealistic/inflated expectations about applying it to mathematical finance, which unsurprisingly came to nothing in the end. I suppose in part because he didn't know much about it. It is a pretty obscure branch of applied probability that enjoyed a brief wave of popularity in the 90s, and quickly went cold again because, as it turns out, the area has formidable and (thus far) largely unsurmountable technical difficulties. At any time this area has had a handful of people working on it worldwide. I think with some breakthroughs it could become a useful area, but nothing like this has happened so far.
I learned later from other people (I was pretty naive about how academia worked) that faculty may try to use their students to try to learn about new areas. I've no idea how common a practice this is, but I have no reason to believe it is uncommon. I recall that the people I talked to seemed to think it was quite reasonable. I think it is wildly irresponsible.
In any case, I suggest you don't let your advisor do your thinking for you. If you like the field you are working in, and think you can be productive in it, then by all means continue. However, if you think your advisor doesn't know what is going on, it is quite possible he doesn't. Researchers are only flesh and blood, they are not omnipotent. If he doesn't know what is going on, he will probably be of little help to you. It is possible for your advisor to learn about the topic alongside you, but you are the best judge of whether he is doing that. If he doesn't seem to be, he probably isn't. And frankly, it is not a good thing for a graduate student to have an advisor who views his PhD as a learning experience. Ideally a PhD topic should be well within the advisors area of expertise. If he does not seem inclined to go into details and talks about generalities, that is a bad sign, in my experience.
Bear in mind that finding a auxiliary advisor who knows more about the topic is a possibility, but if want to go in that direction, I'd do something about it sooner rather than later.
Other people have commented here that it is reasonable for you to end up knowing more about the topic than your advisor does. That is a fair point, but that should not be the case at the outset. Maybe that is Ok after a few
years of you digging into the topic. And really, it depends on you. If you are comfortable going it mostly alone in the topic, perhaps communicating occasionally with other researchers, then that is Ok. If you are not, then it could end up being a bit of a baptism of fire. We don't know what your topic is, exactly, so we can't say anything more about that. And research topics vary wildly in level of difficulty, of course.