You could try pushing that first nap a bit later.
Can you record the nap lengths and night lengths for a while? Just wondering if you have a bit of a longer OT/UT cycle going there with those sometimes much longer naps.
I think if you were to do early BT, it would have to be about 5pm. You would almost certainly be up at 3:30 or so for a resettle though :-/ Is that a time you could show him outside and tell him its still night time and he has to go back to sleep?
If its working for you and he's not cranky and horrid in the early evening, I'd be inclined to leave it and just know you may have to revisit in a month or two.