Congratulations on so many teeth! What a milestone!
I would do the following (and please remember I am just a fellow mom):
I would not have his A time exceed 2.45 yet. This may help him with naturally taking less sleep during the day too.
I would see if it does reduce his day sleep and if not,I would start capping his day sleep to say 3.5 hrs (there are recommendations to curb it to 2.5 hrs, but I would feel its pushing baby too much and may be problem is solved before). I would go slow, capping 30 min every few days to see if it helps.
For this I would do this way:
If your son is a very predictable napper (he sleeps roughly same length of time each day at morning, and the second nap), I would cut morning nap. And I would be consistent, choose the length that makes sense for the second and catnap totals adding up.
If like my daughter every day is different, I would let the morning nap run it's course (up to 2 hrs though), then limit second nap so that baby is up in such a way that 1) he has time to have a last catnap of 30 min or so 2) his total daytime sleep doesn't exceed 3.5 with catnap. 3) he can be up the last stretch of time for less than 2,75 hrs. My daughter lately can get tired before, but can stay up this long. (he can be the opposite and may need to get on a tired side before night). I also feel this changes a lot for her.
I would not let him sleep long during his catnap (30 min about) and would try out if waking him by 4:30pm fits into your schedule.
This is what I wonder might be going on: a long A time leads to tiredness and too much day sleep, at times too long last nap too and close to the night. He may be also then going from OT to UT for the night. And after that he may need exactly ten hours to get rested for the play day. So much more fun!
I would want to check if he is not getting used NOW to feeding at 4:30 because he has done it for some time. (but I would do it after I made other changes). It doesn't seem so, but just to keep on the back of your mind.
Good luck and keep us posted!