H D.:
May I ask if he is drinking too many sugar drinks an hour before the nap? If he is given anything sweet, then of course he won't take a nap. He's too sugared up. Try giving him less if this is the case. If this is not the case, then let him be. He may be ready to engage in real world activities and not feel sleepy.
The other issue is he may have more than enough sleep from nightime (10 - 12 hours an night). He is so energized that he can stay up longer now. For instance, look at the time frame between his morning wake up and the time you try to lay him down for a nap? He should have at least 7 hours awake time before he naps since he sleeps 10-12 hours a night. Therefore, if he wakes up at 7:00 a.m., he should not nap until 2:00 p.m. or maybe 3:00 p.m. and then only for 1 hour should he be allowed to sleep at the most.
Lastly, if there are other kids around who are up and about, he may be thinking they are having so much fun so why go to sleep now.