Does this happen at night, too or just for naps? My first thought is just to leave him, unless of course he is crying like he needs you. It's pretty remarkable that he'll just hang out in there without fussing. I think if you did that for a few days, the "novelty" would wear off and he would sleep. Also, what happens after the 30-50 minutes of being quiet? Will he sleep then or does he want to get up?
Some of the reading I've done says that if the baby spends the time in his crib, even if he doesn't necessarily sleep, he's learning that this is naptime. And eventually, he will use the time to sleep.
Another thought I just had is what is his wake time? Perhaps it needs to be increased and he would fall asleep more quickly. Just a thought...
Sorry to be so random! I am definitely not an expert, but thought I'd throw some ideas out there. Let us know how you're doing!!