Hi there, yeah I wonder if he doesn't need more of a push in the morning. Just capping the nap doesn't always do the trick. I'd push that first A out to 4 hr at least for a few days and then give it another 15 min or so. With the push in A time, you can let him sleep up to 2 hr for a nice restorative nap and then start cutting the pm nap so it's more of a CN to preserve BT once the EW gets sorted.
So, something like this:
W 5:30 am
S 9:30-11:30 am
S 3:00-4:00 pm
BT 7:00 pm
Unfortunately, you can't do much about the heat and the birds which are likely playing a factor as well. Could you try a big box fan in his room to circulate the air a bit and provide some white noise?
Teeth are also notorious for playing a part in EW. Even though he doesn't seem too bothered, they could be enough to prevent him from going back to sleep. You could try a dream med (kind of like a DF, where you don't wake him up, but just go in and squirt the meds in) at around 2/3 am. He should still be sleeping pretty soundly then and the meds may help him continue sleeping when he hits that lighter stage of sleep around 4/5 am. I've never had the guts to try it, but I think it's always worth a mention because I know it has worked for others.