Is he gaining weight well? If so, then you don't need to worry about him getting enough milk. Mine were quick feeders too, and I also found that at around 10-12 weeks they went from feeding for 15ish minutes to about 5 all of a sudden, and they would get frustrated if I tried to make them eat more. I don't think that what you can pump afterwards is particularly relevant with regards to whether he has had enough - you might just be producing more than he needs.
As for overnight I think it is quite normal for them to manage less than 5 hours at a stretch at this age, so I don't think that is an indicator either. Their tummies really are very small, and bm digests very quickly. To give you an idea of how small his tummy is, look at his fist - that is the size of his tummy, so you can see it really doesn't hold very much.
Do you want to continue bf'ing? If you do, then this is just a hurdle that we can help you get over. It's not always a smooth journey, but if you want to do it then it can be done. If you want to move to ff, though, that is entirely your choice and there is plenty of support on the boards for that too.