I am the mother of an addict, however my son is much older than your son and has been at this for years and years.
What helped me find my balance was meetings, Al-anon and CoDA helped me more than I can ever say.
Your son is young. I often suggest (regarding older addicts) that "we" are not the solution, and that living at home is not their only option. Your son is only 15 though, so it may be good to set boundaries and make them clear when he returns.
Take a read around, especially the "sticky" threads at the top of this forum, and you will find a lot of helpful information.
Again, welcome to SR.