My love interest is in a serious relationship with his long time girlfriend. They love each other, but he thinks his relationship (and his girlfriend) are preventing him from being "free". How would he rationalize cheating on his girlfriend (who he does love) with multiple women? How would his girlfriend rationalize staying with her boyfriend despite his indiscretions?
Torn in Tokyo
In my experience with clients who have done this very thing, men don't need a whole lot to rationalize. They can compartmentalize a relationship, and believe to their core that they love one women with all their heart (including having children with her), yet can be sexually involved with others b/c it's "just sex." You didn't mention if he cheats all out or not, but he could rationalize that she doesn't meet all his needs or that she makes him feel trapped with talk of marriage. He can escape his current life circumstances with these other women.
Now your guy's girlfriend is who I'm worried about. If she is rationalizing staying with him, she's in that to-be-pitied place many women find themselves in...stuck with an emotionally (or physically) abusive partner and unable/unwilling to leave. Arguably, her staying with someone like your love interest would be codependency in action. I did a post here about the top 10 reasons why a woman wouldn't leave a situation similar to what you're describing. Maybe it'll help you out some, as well as this post, which is on dependent personality disorder, which can be misconstrued as codependency.
My main character is a recovering alcoholic--in high school. He's been in AA for over a year, avoiding old friends/habits and trying to stay busy. He's a very social kid, which was what got him into trouble in the first place. How likely is he to lie to friends (or strangers) about his sobriety struggles? What weakness might take him by surprise and tempt him back toward bad behavior?
Sober in Summerville
High schoolers are high schoolers, even if following the 12 steps of AA. They aren't the most mature of individuals, though a year of sobriety is nothing to sneeze at. Most addicts I have worked with (who attend AA or NA programs) are very up front about their addiction, as honesty is highly valued within the organizations. You have to be honest with yourself first, admitting you have a problem. This is Step One, and it also reads in the Big Book (for AA) that admitting you have a problem to others enforces the issue. You'll have to decide if you want him to secretly practice sobriety or not, but in reality, those in secret rarely are successful.
As to what weakness might take him by surprise and tempt him back toward using...definitely a love interest. I've seen it happen a few times just in the past few years. When an old flame--still in the throes of addiction--beckons their finger, the sober person tries to help them, and in the process slips up. They might even chalk their involvement with the person up to Step 9 (making direct, personal amends whenever possible), but usually that's a front. Hope that gives you something to think about.
Post them anonymously below, using monikers like Sleepless in Seattle.
I'll get to them in future Dear Jeannie columns.