If your only complaint with him is he smokes weed with his friends then be thankful.
Now if he wakes up, takes a bong hit, goes to lunch and smokes a j, then has a bowl for the ride home to prepare for the nights festivities (getting stoned) it's a problem. Can you say intervention time?
I am going to guess that you are a young couple, if so, he is still experimenting and enjoying his youth with some of his friends. This does not mean he is a "drug addict". Nor does it mean he does not love you or being with you, he just wants some escape time. In a long term relationship this is important
Trust me, as your relationship (and each other) mature things will change. I have many friends that partake in the good herb on occassion and we are all successful professionals.
If you are concerned about his health, buy him a vaporizer and some KB.