When we switched to the big-boy bed (he was just 2) there was a 3-week spell of reluctance to nap. I didn't give in and returned him to bed until he had been in his room for 1 hour. He started napping again, and has napped consistently until now (he is 3 1/4) when he is becoming inconsistent again, but still naps a couple of days each week. We have a 5-month-old, also, and it is a beautiful thing when they nap at the same time! It doesn't happen every day, though. I still enforce a quiet hour on days when the preschooler is not able to sleep -- he can look at books or play with stuffed animals, but nothing loud and he has to stay on his bed. Many 3-year-olds are hit-or-miss with naps, but before the third birthday most still manage to nap in the afternoon. I think the trick is not to reward the behavior with either positive or negative attention when they get up. Keep it boring, return him to bed with minimal conversation, and good luck!!