Oh great......My 14 month old son is now resisting his nap. Need advice.?

My son went to one nap a day at a young age, like 9 months old. He was been consistently talking a good 1 1/2 to 2 1/2 hour nap. Recently, he's been resisting it and playing in his crib for over a half an hour before taking a shorter nap. One day, he didn't even take a nap at all-that was a "fun" day. I know at this age he NEEDS a nap. Rocking him to sleep doesn't work because he's basically out grown it. Should I start waking him up at a certain time in the morning? I've been letting him sleep as late as he wants. He generally sleeps around 12 hours a night. Waking up once in the night, usually. Any helpful advice is welcome.