S.L.
It's a stage and unfortunately, letting him cry it out only works if don't get them. If you get them before they fall asleep then you haven't actually let them cry it out at all and he's gotten exactly what he wants.
The fact that he starts playing and is fine with you tells me that he just doesn't understand and can't yet respect that sleeping is to be done at night. Think about it this way. He doesn't have a job to go to. He can take a nap any time he needs it and he instinctively knows that. So it's no skin off his nose to be up and he has no reason to be able to understand why you won't cooperate with him.
As long as you know that he isn't hungry or sick or in pain from teething or something else like being scared of the dark etc.. Then you just need to let him scream til he gets sick of it and goes back to sleep. But that only works til they can't clime out of bed! :)
Hang in there. This will pass.
Suzi