I would say more likely OT - he's not getting an excessive amount of day sleep and his nights are 10.5h which always happens here when DD is OT. I wonder if your first A is too long for him at the moment, and that's actually giving you an OT nap, which is why the second A is so short? Would he go down a bit earlier, say 3h30?
Whatever you decide, A times or set naps, you need to stick to the same thing for a week to see if its working before making more changes.