I think it was unfair of his mother to ask you to report to her about anything he does. You are very young and inexperienced in life and are finding your way, and in my opinion it was unfair of her to ask you to report his drug use to her. I can't help feeling she is using you. I know she's worried about her son. But to ask you--his young girlfriend-- to carry secret information back to her is, in my view, unfair.
If he's an addict he'll be lying to both of you everyday anyway.
Your words--"I want to help him and be there to support him"--these are the words of the codependent partner of an addict. So you are in the right place, here at SR, and you will gain a lot if you read the section of links labeled "Sticky" at the top of the forum page.
Take good care, dear. You are in dangerous waters.