I really didn't understand what the "sub act" was, I'm sorry.
If you meant a sub-account, then you must be the Primary Acoount Holder; as the adult, since he was minor when he got that account. Legally he's no longer a minor now.
However, as the Primary Account Holder, you get "Administrative Rights" over the other users.
Call Yahoo, or e-mail them with the details.
They'll reset the password for you and you can then create a new password for him which he can't change without your permission. They will not, however, give you the password as that would be a violation of his Rights.
All the best.
I do hope it was sub-Account, that you meant; otherwise, please disregard this information.
Please do not install any keylogger, because it's the motha of viruses! And please don't try hacking, because that's illegal and you don't need to do that.
I think you should also consider what kja63 said about opening lines of communication and trust with your son.