I personally would think that a 1.5hr nap (if it's the only nap all day) is not enough for an 11 month old.
At 11m there shouldn't be only one nap, 11m is only the beginning of the transition to 1 nap, or the middle of it for few babies.
If you have a 1.5h nap + a CN then it's ok, IM
Siobhan, I actually think you did the right thing giving him a CN. This transition to 1 nap was the hardest for us and you ought to get many messes up days. The A time before your CN (the shorter nap whenever it is, AM or PM) doesn't have to be a full one, because you are aiming for an Ut nap anyway.