being a guy, this is still a mystery to me. But nothing gets me going more than light scratching.
Try scratching lightly on the chest, and in the stomach area. The key here is making a sort of tickling affect, his stomach should recess, and come back up as fast, usally followed by a heavy breathing.
backs and tops of shoulders, are great too, and even the top and sides of the head.
A guy will let you know almost automatically if it does anything, due to its unexpectancy. But i will say one thing, it drives me nuts! 
Not so much "tickled" as lightly running your fingers over someone's body just enough to cause "goose bumps"... mmmm...lol.
my b/f loves to be tickled as in i just run my fingers and hands lightly across his back.
its not even really sexual for him it just relaxes him.
he wakes up in the morning saying, " i need tickles" lol.
Do all guys like that? Some guys hate it if they are tickled, though.
Should I be in the other room scratching my man? he he
You're not alone. I really like light scratching as well- up to and including leaving a mark....lol. Just try not to draw blood.