hi i would guess one of two things.
Obviously as he gets older he can stay awake longer, so maybe hes not as tired and could stay awake a bit longer until his nap, watch for his cues, at 12pm if you dont think hes tired let him stay up a little longer before you put him down. OR you could try an earlier bedtime, i would say 6.5 hours A time before bed is a litlle too much, but i would more say it may be that hes not as tired at 12pm.
My lo is the same age as yours, and has just gone to one nap, his rountine looks like this
6.30am awake and bottle
8am at childcare, brekfast
11am lunch
11.30am nap (about 90 mins)
5.30 home and dinner
6.30 bed
I wouldnt try and get him back to 2 naps a day, he shouldnt really need two now, but your his mum and you know best, you just need some advice, think about it and really watch his cues, he can kinda tell you what he wants, i find now that when my lo is gettin tired, he follows me around and keeps grabbin me.
good luck
tracy xxxxxxxxx