i agree with pp. at this age LOs need a lot of sleep during the day - even as much as 6 hours in a 12 hour day. usually this is 3x1.5 hours and 1 45 min nap during the day. awake time at this age is only 60-75 min usually. try putting him to sleep earlier, if you miss the window it can be harder and they resist. as soon as you see a yawn, even if he hasn't been awake very long head for the crib.
are you familiar with the 3 hour EASY routine in the baby whisperer books? how do you try to get him to go to sleep?
also, he may be snacking every 2 hours which won't help the naps, try to stretch him out to every 3 hours for feeds, this may help the sleep as well.