My son also accepted his big boy bed, and then refused to nap after a few weeks. I thought he was done with his naps, and really was distraught, since he's the kind of kid who is full throttle all day long and NEEDS his naps. I really just stood my ground for about a month and he finally relented. He's now back to taking 2-2.5 hour naps every day (except on Sundays when my hubby's home during naptime and he doesn't want to miss out on precious daddy time.)
It was rough...there were a few times he would sit in his room and scream at me, and kick the doors. VERY frustrating since he'd do this at the same time as the baby was napping. I truly think he was just testing me to try to find my breaking point, but he never found it and I won. The no emotion, no talk approach is the best. Keep at it, mama!