Asperger's and Humor

My son, who has Asperger's Syndrome, tries hard to tell jokes. I'm not sure it's a natural skill for people with Asperger's Syndrome, due to high incidence of pragmatic language problems they tend to have, but kids with Asperger's can do some unexpected things very, very well, so I don't want to generalize.

My son, however, is so bad at joke telling that at home, the actual "badness" of the jokes are often terribly funny.
Jokes he came up with on his own include these:
Your mamma's so greasy she eats bacon.
Your mamma's so fat, she's fatter than anyone I have ever seen (told to a boy with a very heavy mom).
Knock knock
Who's there?
The mailman
The mailman who?
I said, "The mailman." Stop it. You're trying to mess up my joke.
And he overheard Sarah Palin's speech last which had a joke in it that went like this:
What's the difference between a hockey mom and a pitt bull? Lipstick.
And this morning, over breakfast, he retold it like this:
What do you get when you cross a pitt bull and a hockey stick? A pip-stick.
We wanted to help him become better at telling jokes and fitting in socially, so we decided to teach him several jokes that he could memorize and just pull out of his hat when he was in a joke telling mood. Humor is an important social skill. So, we taught him several "yo mama" and knock knock jokes, and animal jokes, and had him practice telling them. It helps to have a repitoire. So what if he memorizes them like math formulas -- if he gets the setting and timing right ( no small feat in itself), then no one need know he memorized the joke?
This doesn't seem to be enough. So many jokes are about repetition and timing and nuance, all concepts that are elusive for my son, that, even though he has some decent joke ammunition, he falls flat in delivery.
During a conversation among adults at family party, the talk turned to a terrible police shooting in our community. Someone said the word "road" and since my son figured we were at a party......he piped in with: Why did the Chicken cross the road? With a punk rocker and a stapler!" Forgetting for a moment that the joke goes like this, "How did the punk rocker cross the road? Stapled to the back of the chicken," my son was so far off in reading the conversation well enough to understand how and when a joke would be appropriate, that most of the people at the party thought he was on another planet. My husband and I heard the trigger word ("road") and knew he was trying hard.
He is finding his way, however. A few months ago, he told his oldest sister that she looked like Chuck Norris (she does not). She laughed in surprise, and that was all he needed. He began to find ways to remind her of how and when she looked liked Chuck Norris. He does it several times a week, when she least expects it. Here's one:
"Dude. Sit up straight."
"Oh yeah? Well you look like chuck Norris."
And another, when the sister was babbling about something:
Mom, will you please tell Chuck Norris to be quiet?"
And today, just before dropping him off at school, his sister was in the front seat and she said, "Bye, Buddy."
And he said, while getting out of the car, "I just thought you should know ... your braids look like Chuck Norris."