It kind of sounds like a cop out answer but it really does depend on the guy. I mean most guys are sensitive in the same areas but to a different degree. And even if they are sensitive in that area, he may not like it touched.
Like a guys balls are really sensitive and some guys love having them handled and some don't. A guys inner thighs are really sensitive and some guys love having them kissed. Try giving a guy a hickey there! Guys like having their hands kissed and their fingers sucked on but I don't know if that's cause they are sensitive or cause of a mental thing. Also, some guys like their butt touched or grabbed and others will freak out, like you are trying to turn them gay!
But the lower abdomen and the neck seem to be really sensitive for a guy and I can't say I've known a guy to get bothered by you kissing either. They really seem to love it. Kiss down the lower stomach to his penis or up the neck to his ear and you will probably drive him crazy!
But you are always safe just touching his penis. Some guys don't want to be touched or kissed all over, they just want their penis touched or kissed since it's so sensitive.
So it really depends on the guy but you can have fun finding out what that particular guy likes!