It's generally agreed that rat food from pet stores is crap. While I do use this myself I also supplement with table scraps and small bits of fresh, raw fruits & veggies. It seems to work well for me

I am still a noob to rats though but I found a link that may be of help http://www.ratfanclub.org/skin.html

From the last section:

"There are two main causes of bald spots in rats. The most common is barbering, a behavior where a rat obsessively grooms itself or another rat to the point of nibbling off the hair. The result is bald patches or areas where the hair looks like its had a bad haircut. Usually there is no damage to the skin, but sometimes there can be scabs. The most common areas for self-barbering are the front legs and stomach. The most common areas for barbering another rat are on the head, face, neck, and shoulders. These bald spots are not usually symmetrical. Because this behavior doesn't usually cause any health problems, there is no reason to separate a barber from her roommates, unless you are showing your rats. Another cause of bald spots is fungus."

My sister has a dog that exhibits this barbering behavior. Usually it's caused by flea bites or another irritant. If you catch your rat chewing at himself I think it's likely that's your culprit.