Pixel, it's unusual, but it does happen... when I got Tom from a shelter situation - he'd spent nine months being bounced around from pet store to humane society to pet store in a small cage with his sister - he wouldn't groom himself. His coat isn't all that great to begin with, but it gets downright greasy and grimy when it's not groomed. I think it could have had soemthing to do with him being depressed/stressed/scared. I really don't know. It DID come back to him, though, after maybe a year with me. He always grooms me and the other two cats, but he seemed to have forgotten all about grooming himself.
If I were you, I would give him regular baths and grooming. You can use dawn soap as a de-greaser, but I wouldn't use it regularly. I love anything by Earhtbath, the mango tango smells especially good and keeps white coats quite white. He'll probably need a once-a-month or more bath depending upon how dirty he gets himself, and I would do a brushing every couple of days. Buy a few different combs and keep them where the two of you cuddle most, and catch him while he's relaxed to brush out problem areas. You can also get kitty wipes if he needs to get clean but you don't have time for a bath. A kitty companion, if he's friendly towards other cats, might be nice for him, too.