I agree with the other ladies. Lie him down then leave the room, leaving longer each time. They do get tired of it eventually. Don't smile back, you don't even have to say anything, just lie him down again each time.
Once they are a bit bigger and doing it for attention just leave them. Unless they are distressed or climbing out then they will get bored and go to sleep.

I didn't have as much trouble with DS1 standing up as I did with him climbing out of his cot and walking out to show us how clever he was. The same thing worked though...just keep putting him back until he can't be bothered doing it anymore. I thought I was going to go crazy a few times but he stopped doing it eventually.