S.M.
I went through the exact same thing with my daughter at 3 months. She was the biggest cat napper there ever was! I read a book called "Healthy Sleeping Habits: Happy Child" and I now have her on a wonderful nap schedule--3 times a day. We started at 4 months old and it's been 1 1/2 mo. now of her sleeping beautifully! I would recommend to read the book for ideas, my other thought is just put him in his bed for his naps and (although it breaks your heart) let him cry until he learns to self-sooth. This happens within 3 days. The book recommends that they don't stay awake in the morning (after waking up from sleeping through the night) any longer than 2 hours. So, my daughter gets up around 7/7:30 and she is always back in bed no later than 9. Then, they should sleep about an hour or so, when they wake up you again have no longer than 2 hours of wakefulness. So my daughter would be up at 10 and go back down no later than 12. Understand what I'm saying--it sounds so confusing I know!? I would read the book, try to get him on a napping schedule and eventually he will get used to it, it'll all fall in place. Good luck.
Oh--and a sound machine works wonders b/c they start associating that sound with sleep!