Middle Son loves to be tickled. Even now, as his 12th birthday approaches, it's one of his favorite forms of play. I know another father who's autistic teenager loved to be tickled. While typically developing children outgrow wanting to be tickled, lower-functioning autistic children apparently do not.
He informs me that he wants to be tickled by leading me into his bedroom, lying down on the bed, and pulling up his shirt to expose his tummy. Then he tries to grab my hands as I move in to tickle him. He also tends to pull his feet up and block with his legs. He's big enough now that I'm uncomfortable with that--if he inadvertently kicks me, he's strong enough to do some damage. So last night, for the first time, I started telling him "feet down". I use "hands down" in various circumstances, and I also say "feet down" when he is rocking his chair too hard at the dinner table. (Actually, the dinner table chair is probably worthy of a post of it's own).
I wasn't sure if he would respond to it, but he did. By the end of the evening he was mostly leaving his feet down when I tickled him.