There are several things to consider or try;
*Look into diet. Believe it or not, diet can be a major contributor to irritable behavior and even lack of sleep.
*Up his physical activity in the morning. Maybe he's not getting enough exercise to tire him all out. Get him to run around in the yard, buy a soccer ball and make him volley it as you and he walk to the nearest park to play with it, and make him volley it back home. Eat lunch then nap.
*Some kids need soothing instead of more activities. Perhaps a warm bath, warm milk, room darkening blinds or curtains, and some sort of white noise or soothing sounds (like a nature cd) or classical music.
*Some kids do outgrow naps, and then going the "quiet-time" route is a good alternative. Try stretching or mat exercises to help him relax and settle-down. Read a quiet soothing story to him, or just ask him to lie quietly on his bed for at least 20 to 30 minutes. Allow him to look at books or to color. Avoid tv if you can. It's too stimulating. Even more mellow kids programs. If you encourage picture books or just having them lay still with their eyes closed, sometimes they go to sleep on their own because the pressure is off. At the very least, he will wind down, and maybe learn to self-soothe and relax on his own.
Last but not least, check out "Sleepless in America" by Mary Sheedy Kurcinka. It is a book about kids with trouble night sleeping, but I think all of her recommendations can still apply here. The book will go into the diet and recommendations if you're interested in what to do there.
Good luck!