Poor little guy. I have a lot of personal experience with sensitive skin, rashes, and hives. Any ointment, like A&D or petroleum jelly, will work to keep irritants out. Because of the location, you may have to reapply it regularly.
To actually calm the irritation, if you have easy access to a health food store, look for calendula cream in a tube. Very, very soothing and healing for all sorts of skin irritations and rashes, although if this area is really irritated, the first application could sting ever so slightly. A company called Boerke and Tafel makes a whole line of medicated creams and ointments, and I find them all wonderful for everything – burns, rashes, insect bites, hives, sprains – the homeopathic ingredients are non-irritating and wonderfully effective for me. An multi-purpose favorite of mine is called Florasone; it works much like cortisone cream, but in minutes instead of hours.