Letting the Funny Leak Out

Nathan likes telling jokes (and in the wake of Robin Williams' suicide and a slew of comics telling us how telling jokes is the sign of a damaged psyche, I worry).  However, he doesn't quite understand the subtle difference between a well-delivered punchline and simply shouting out random phrases. 

He sees Daddy get a laugh by shouting out "Interrupting Cow" during a song.  He sees Mommy get a laugh by telling a knock-knock joke.  The Lego Movie got lots of laughs with "Honey, Where Are My Pants?" but he doesn't seem to understand the timing behind it.  Simply shouting "cow" or "pants" or endlessly repeating the same knock-knock joke isn't funny.

I've tried a new tactic: I told him that a joke is only funny twice.  After that, all the funny leaks out.

I'm hoping I can use it as a short-hand to curb the behaviour without having to be mean and telling him to stop.  Before this, he's persisted even in the face of repeated requests to stop and kept going right until irritation shifts into anger.  I hate to hurt his feelings but he's got to figure out when his behaviour is inappropriate.