Just saying if somebody - even my best friend - did that to me I'd flip shit and probably slap them in the face...
But I'm glad you two are closer than that...
If you want him to tickle your feet, try this: if you're ever sitting and watching tv or a movie, put your feet in his lap and ask for a foot rub, somewhere in the middle of the foot rub start giggling and pull your foot away, he'll get the hint and probably want to keep going.