Hi @slowtogo, welcome to the forum 
Have a chat. You wont know if you dont ask, he might be really into it.
Watching him masturbate or mutual masturbation is a great idea. If the whole bum thing puts you off watch his reactions to it. The way guys react to that kind of stimulation can be incredibly erotic.
A nice idea might be to get a vibrating butt plug with a remote. He can put it in, you can have controll of the remote. They feel amazing and are a Great way to participate for a partner. Fun for watching him masturbate or for use when your having all types of sex (fun for when a partners giving you head too).
(Ps. A couple of threads you might find interesting
Pegging, do women enjoy pegging their man?
Pegging, why do you like it. From all perspectives )