Question for parents of teen age boys only please.?

My son is almost 15 years old. He is a good kid, involved in 3 competitive sports, has good friends from good families but.... he and his friends have started to try alchol. It started about 7 months ago to the best of my knowledge. Another factor here is that his dad committed suicide about 18 months ago. Now I know this is not a reason to do what he is doing because the other boys are not in his situation and are doing it. He swore to me the last time he was caught that he had stopped. He will tell me when his friends do it and that they need to make better choices. I thought things had changed with him until yesterday when I found a small airline size bottle in our trash can. He had friends over the day before and swore that it wasnt his that it must have been one of the other boys. We went back and forth for several hours about if he was still doing it. Finally he said that he had done it a few more times since he had told me he stopped. How do I deal with this? He is in counseling and I know that it is a peer pressure thing that he has to learn to deal with but it is tearing me up inside...he is better than this. Its not every day or even every week, I keep a pretty tight reign on him. I'm glad he finally confessed and was honest with me but I'm not sure how much more I can control his life without keeping him on lock down and not allowing others into our home. I don't have any alcohol in my house anymore due to this very reason. Someone brought it with them. Any advice would be great!