I didn't have this problem. But this is what I would do. Keep them in a very light and bright room during the day. Going about your business and what not. Keep them awake. Then only at the time you want them to nap, darken their room as much as possible. Other than that I would talk to the pediatrician about ideas or info.
Keep him up during the day for about 3 hours at a time, during nap time keep the room lit, when he is awake keep him "active" talk to him, play with him, read to him, and night give him a bath talk in a lower voice. It will probably take about a week but, he will get his days a nights straightened out.
I woke mine up during the day at least every 3 hours to eat, and then let them sleep as long as they would at night. It didn't take but a week or so for them to start taking their longer "naps" at night.