I think that he has earned a full credit already, due to the amount of hours he has put in. There are two ways to figure credits. One is by finishing most of a textbook. Two is by counting hours. If he had kept an hours log, one credit could be awarded after approximately 180 forty-five minute sessions. Probably less than that, actually, because that is equivalent to perfect attendance in public school with no days taken off for assemblies, etc.
I would advise against putting a one-half credit on a transcript in pre-calculus because, in my opinion, that looks better than one-half credit. It is best to have his transcript look as typical as possible.
Is he planning to take calculus in college? If yes, then you may want him to finish the book:).