Leela
do not use any soaps or shampoos, they can leave a residue on the fur that then could be ingested when he grooms himself.

Leela
I just saw the pic. Yeh he is intact and that def needs cleaned. Just use warm water and gently rub his head. I would just keep him in a pouch and clean it while he is sleeping if you can. Keep an eye on it over the next couple of days if it doesn't seem to get any better with regular cleaning than you may need to take him to a vet.

Leela
That would be his "bald spot" I'm assuming he is intact ( not neutered ) The "bald spot" is a scent gland that produces an oily like substance that sometimes (I've heard) gets "crusty" Without his partner to help groom him He may not be able to clean it well himself. You can take a dampened paper towel with warm water to soften and clean off the crusty bits for him but make sure he is completely dried off before putting him back in his cage so he doesn't catch a chill. DO NOT use a hair dryer! Just rub his head with a dry washcloth or something soft until his fur is dry. You might consider having him neutered before getting him another cage mate ;) He will need a new friend and some time to get people friendly. You will have to invest some time and effort into bonding with him.