You've gotten some great advice already, so I will just suggest: 1) a shorter nap, 2) a predictable bedtime routine with relaxation built in ~ bath, brushing his teeth, putting on his pj's, a story or two, soft music & voices, and 3) a snack an hour before bedtime to relax him and induce sleep. A small serving of cottage cheese or a warm glass of milk and whole wheat crackers or whole grain toast with some cheese or peanut butter, a small serving of low-fat ice cream, a banana or a banana smoothie made with a banana, low-fat milk, and a few ice cubes (the calcium will enhance the sleep inducing effects of the minerals in the banana), a small bowl of oatmeal with a little milk or a couple of oatmeal raisin cookies with a glass of milk are all good sleep-inducing snacks. Make sure the snacks are light or they may interfere with his digestion and keep him up, which you don't need.
Hope he gets back to sleeping easier soon!