Either hereditary baldness, or he's had it shaved for charity, or he likes it that way, or he wants to be a hard case ... why don't you ask tim, it's probably not a secret!
Forgive me Jon Boy if I'm wrong but I believe that your question is not serious and does not actually deserve the courtesy of a reply. If as you say Tim is your mate you must be close enough for you to ask him the reason why he is bald. If he doesn't know, and you don't know, then the strangers at Answerbank have no chance of knowing.If your question is a serious one then please do me (and possibly others) a favour by giving more detail concerning your reason for asking the question.