Ahhh...okay, well that should be about perfect for his age then- the 45 min A time.
Hmmm...is he a refluxer? Cold/hot? It might take some experimenting to see why he is so uncomfortable with sleeping. Is he a refluxer? If so maybe doesn't want to be on his back?
The other idea is to extend the naps as you mention. You could try the nap board for help with this using pat/shhh. It could be that once you get the naps extended he won't be so overtired, but if he is sleeping only 45 min at a time he's going to compound the tiredness, it will build on the last because it is my understanding that it is the second 45 cycle (1.5 hr nap) that is the deep one, so if he only has the first cycles he's not going to get that "restortative sleep."