Thanks for your reply.
It's honestly not that I'm only letting him have 2hrs A time - he's rubbing his eyes, giving out, etc, if he's awake any longer! He's generally awake 2.5hrs in the morning before 1st nap, but he can't stay awake any longer than 2.25hrs before lunchtime nap (and he's still napping for 1.5hrs).
As for length of the morning nap, he would never ever nap for longer than that, despite lots of tweaking with his routine :-( When we've kept him awake longer, he still naps for only 30 mins or so, but is very difficult to get asleep as he's overtired.
So if he drops the third nap, he'll have 2hrs total naps a day, is that enough?