I wouldn't worry too much about it.. My son does the same thing with his xbox and so does my nephew.. i agree about them just workin it out outloud rather than in their heads.. We ended up getting Xbox live and he plays games with other kids online.. alot less of the talking to himself now..he has alot of friends to keep him occupied and talkin to them..
Some people work things out mentally and others do it orally. Ever heard pilots do a precheck before taking off? I don't think it is a problem. If he starts doing it at school or something like that, then I would be more concerned.