This is all so hard with such a young kid. I agree that "snooping" is only going to drive you crazy, but at the same time a kid his age needs some monitoring. The trick is to find the line between monitoring and and invading his privacy.
I have a 15 year old non-a daughter. I have all her passwords (to facebook, twitter, etc) and she knows I can read her texts if I choose to. I do not. I do monitor the bill to make sure she's not using the phone inappropriately (too late at night, during school, etc). There was one time I found she had been up texting 'til the wee hours.. we had a discussion and agreed that for one month she would bring her phone into my bedroom at a certain time at night. She did so willingly and after the month was up we never had that problem again. I am "friends" with her on facebook and if I see something that might be questionable I talk with her about it. I find that as long as I do so in a non-threatening/non-accusatory way she will usually end up agreeing with me (because, after all, I'm always right - lol) and will take steps to rectify whatever the issue was. If she wasn't so agreeable I'd have to consider if I could trust her to have the phone or access to a computer in my home but THANK GOD.. so far so good.
Things were not so easy when my AS was 15. I started off with just the "monitoring", but when it became obvious that he was up to no good I initially did the snooping thing -- and drove myself completely insane. It did nothing to change his behavior at all. If anything it made things worse because it escalated the power struggles between us and made him even more determined to get away with things. I finally decided that the phone was a privilege, paid for by me, and if I didn't approve of how he was using it he would not have it. That solved the problem.
Your son seems to be trying to do the right things. Much of what you found by reading his texts is regular teenage stuff that every kid needs to learn to handle on their own. Maybe you could try to not read his texts and just look at the bill to see the numbers he's texting/calling. Let him know that if you see he's calling/texting known drug friends then he will lose his privilege to have you pay for a phone for him.
I found the last thing my son wanted to do was talk with his mother about sex. Instead of trying to have big talks, I'd look for "teachable moments" that didn't make it feel like a lecture -- like maybe when watching a TV show, or hearing some lyrics to a song. At his age he really needs positive male role models to learn this stuff from, hopefully your son has some men in his life who fit the bill!
I'm very glad to hear your son seems so committed to his meetings and staying clean. Focus on that and try not to worry so much!