Hi there,
Sorry to hear things are rough right now. How does your lo respond to early bedtimes? that is what I would suggest tbh at this point. If he is up at 5am I would aim to try having him asleep by 6.30pm latest, ideally 6pm. I think there are a couple of issues going on here. the first thing that jumps out at me is that 3pm-7.15pm is quite long after a 40 min sleep at this age, particularly if he has been up since 5am. I would have aimed to have him asleep by 6.30pm at the latest.
I also think it may be worth trying a short am nap i.e. 30 mins and a longer pm nap but you will need to get him out of the OT loop first.
Is there anyway you can get him back to sleep at 5am when he wakes?
becky x