Above all things, strive to maintain trust and respect. That's what friends do. It's actually far more awkward for a friend to provide that kind of personal care than it would be for a stranger, and if I were your friend, I would decline your generous offer.
Since one or both of you are likely to become aroused in the process, you'll have to have an honest discussion of the subject and follow his wishes. This is one of those situations that we fantasize about, but when actually faced with it, it's not going to be all that much fun.
Good luck!