My DS is the same age...On one nap a day from 1230-230 capped at 2 hours. And he is now sort of dong EWs... ie wimpering or chattering on and off between 5 and 7am. We have chalked it off to teeth for us, as he has been on one nap a day for almost 2 months.
I do agree with PP, that your LO is clearly headed to one nap. Try pushing the morning A time. For a while when we were transistioning to one nap, our nap fell right over lunch time. from about 11-1 or 1130-2 depending. We just fed half the lunch before the nap and the rest upon wake up. HTH