Oh, sorry! I thought I had posted a reply but its not there.
I think given he's now getting more solid night sleep, he may need a little A time increase, say by 15min every few days til his nap lengthens again. Its tricky because at this age, they can have an UT nap and know they still need more sleep. I wouldn't keep himup til 12:30 straight off the bat but just gently increase over the next week or two and see if you can find a sweet spot.