Perhaps the nap needs capping, it's quite long. Leaving it this long means he's not ready to go to sleep at a reasonable BT and so at 7am he is actually tired. Sort of a UT/OT loop, but one he is coping with by making his day progressively later. I would suggest capping the nap the a maximum of two hours and waking him at 7am for a few days and seeing if that helps.
It could be a teething thing making him more tired but with this much sleep he's making it almost a 26 hour day. I think you need to help him gently back into a day with 24 hours in it.