First question, does his story sound right? Can you develop a dependency to Xanax so quickly?
His story is his story, active addicts lie, then they lie some more. If my exabf said he took x number of something, I always multiplied by 3...got me a little closer to the truth.
They will say anything to get what they want and protect their addiction.
Second question, do I need to tell his father? My boyfriend is a 42 year old man and he actually did say that he doesn't want to tell his family about this because it's never going to be an issue again, but I'm not so sure about that. Should I let his father (who he is close with) know what has been going on?
He is 42, not 12, it is up to him to let his family know, not yours. As far as "it" never being an issue again...maybe...but not likely, less than 10% recover for life and I will assume that he is still smoking pot, so he is not clean...and this is not the first time he has abused"pills", this is a repeat performance.
Have you read Codependent No More by Melodie Beattie? if not, I would suggest that you consider doing so.
Keep working on you, get healthy, now is the time. Good to hear that you are not living with him and going to meetings.