It's so difficult to determine as I do think he's prone to OT as most lo's are. I think a couple of days of EBT for him worked but now he's reverting back to his usual 10hrs at night. This indicative of either an OT LO or one with LSN, it's just hard to judge sometimes. Now that he's waking even earlier than before after doing the EBT for 5 days leads us to think perhaps he's LSN as an OT bubba would've started to sleep longer at night by now. See where I'm going here?
If we look back to where he was before he was sleeping 12hrs in 24 when he was on those two short naps. Now, as he's getting that with (before) a 1.5hr nap and 10hrs ONS, or near enough. The 1.5hr nap must be so much more restorative than when on those two short naps. So what I'm saying is that if your kindergarten could go back to the original 12pm nap time, hopefully he'll go back to his 1.5hr nap. Doing that, you may be able to start to push BT later as you wanted to see if he's truly LSN. If he's only sleeping an hour at that nap time atm though, going back to too late a BT might be a little too much too soon iykwim hence I suggested the 6.30pm BT for a few days considering he's waking even earlier.
I think eventually what you may find he'll do is something like:
Wu 6
Nap 12-13.30
BT (asleep by) 8.
It might take us a little while to get there as I didn't want to jump him to that new time too early in case that perpetuates the early WU's in the morning. Wdyt?