I know...it's very painful. Unfortunately (as you've learned) there's nothing you can do to point out the obvious that their kid is still using. Whenever I did this, my bf would become furious and tell me that my "best bet" was to totally stay out of it. In other words, he was threatening me that if I didn't shut up about his kids he'd break up with me. Well...living in denial is not in my nature. There's nothing you can do to make them see what's going on, they will ultimately resent you. My favorite saying regarding this is "No one is blinder than he who will not see". The only thing you can do is step back, and let your bf find out on his own. Eventually his son will do something to him that is hurtful enough that he will finally wake up. My bf did eventually realize that he'd thrown out the best thing in his life so he could enable people who were addicts and would screw him over in a heartbeat for anything they could. When/if that happens, just tread gently with your bf and let him own the problem. Ultimately it's HIS kid...and when push comes to shove he'll be very quick to remind you of that. Remember...in a step family situation (even though you're not married to your bf I still consider this a step situation) and when drug addicts are involved, the first rule is that you must protect yourself. Many times, our "partner" won't because they're blinded by the chance that their loved one has "changed".
I have no tips for dealing with the pain of it all. Like I said, just focus on yourself even when you and your bf do begin to talk about all of this. I found a hobby that I love dearly and when I find myself getting stressed out, that's what I think about. I was seeing a counselor a little over a year ago. My bf came in for one session and wouldn't go back. I really wish that he'd go to counseling either on his own or with me. My counselor said he was totally clueless when it came to drug addiction. He also has codependency issues with his children. But...I can't make him go...I can go on my own to help keep ME focused on what I need to do...but that's all I can do.