I'm popping in for just a day, but my comments are pretty straightforward and you've heard them so many times before.
1) S appears to have a serious problem with true commitment. Intellectually, he can grasp the concept and even admire it, but he doesn't seem ready to accept it. For the moment, it seems his notion of commitment to him is having you on his terms only. As we both know, true commitment, especially marital involves mutual giving and an ongoing desire to give.
2) In this light, when S mentions that his love life is a mess, I suspect as you must know in your heart that this is more about him than about you. His doubts about you--which he must have because he is human--only feed into his confused notion of commitment and cause him to wonder if he's making the right choice. I think there's a real fear of losing his independence (and youth as you've noted) because he has lived a certain life for so many years and is avoiding the natural consquences of your R. Thus, while I believe he loves you, it's as much in the romantic sense than the marital sense and he simply can't cross the divide. Having one of his good FF getting a D can't help his views of the next step.
3) I can't explain why S is acting the way he is with your other FF friend, other than to say that it is a manifestation of his inability to truly commit to you. Basically, he's subconciously or even actively looking to place buffers and obstacles between your R and its future. These seemingly innocent obstacles remove the burden from him from actually addressing where your R is headed. AND YOU ARE NOT WRONG TO OBJECT. If there is a notion of exclusivity in your R, certain old relationships and new ones must take a back seat or even be eliminated for the sake of the primary R.
4) S is an organized thinker who focusses on assets and liabilities. I often advise others to view their M's as balance sheets with many assets and liabilities (please distinguish this from Marriage Builders Love Bank). In a true committed mutual-love R, the bottom line should always be positive because the assets outweigh the liabilities. But make no mistake, there are significant liabilites in terms of giving of oneself, loss of total freedom, and the assumption of responsibilities we just as soon not fulfill. Human nature tends to focus on these liabilites, and for some, it does so to the point where the assets and positive net worth become totally obscured. DBing is as much about managing the liabilities as it is increasing the assets.
I know you don't like to push, but maybe your QT time in Vermont can afford an opportunity to explore more philisophical questions on your R and R's in general--BUT DON'T DO IT IN THE CAR. Each of you needs an escape route if necessary.
One thing I guarantee: you won't get the same reaction that I got from your advice!!!! Be good!
Be good! 
