hmm, I don't know then, Just thought I would ask and give that thought, you never know.
He sounds like a great guy though! You know him best and it sounds like you are on the right track with him and have thought of everything, if your instincts are telling you that it may be time to raise them, as long as you have ruled out everything else, then you could try raising them, it may be that he is ready.
When I was teaching Charlie to jump my instructor had to put the jumps higher so that he would think they were indeed jumps and not trotting poles! Then he made an effort and jumped nicely. He's a big guy, he's 17.1 h, maybe they seemed too small for him!
So yes, you may be right that it is time to raise them for him. You could also book a lesson with an instructor, and that way you will know for sure what to do.