Well of course, I'd be looking at some variations of tickling. This boy is in a perfect position for so many of them.  Aside from the obvious tickle spots, I'd note that this is a good position to add feather/soft brush work on his neck, ears and - hoping that the results don't cause him to hurt himself while in this extreme position - nose. I'd keep the blindfold, which always heightens the surprise factor in tickle scenes. And, for boys that respond well to it, those torturous whispers (including misdirecting his attention to the spots you DON'T attack next) are de rigueur.

I also note that this boy is very clearly advertising that he's enjoying himself in this predicament. Such a good boy you've found! But I am curious - how long do you think you can safely keep the boy in this position without causing lasting low back or other damage?