Hi there.
It sounds like he is a little UT for the first nap, however if you are starting the wind down at 1hr 30 I am assuming sleep is coming some time after this (how long is the full A from waking to sleep?) which sounds like it is probably a good A time for his age.
My LO used to have a longer first A because we were always at home, his own environment, his own toys etc meaning that although he was stimulated I saw it as 'generally' a lower level stimulation than when we were out in the afternoons and going to baby groups etc where he saw more people and had a higher level of stimulation, making him tired sooner. Do you think this might be the case for your LO?
You could try an extra 10mins A time in the morning and see how it goes.
I must say - it sounds like you are doing a wonderful job and I am envious of your 12 hour nights