I've been on Wellbutrin for over a year, so I can only relate my own experience. BUT I have tried most other antidepressants, and Wellbutrin (for me) has the least number of side effects. It would be odd for a doctor to put him on this for his particular problem, because Wellbutrin can actually INCREASE your sex drive, at least for awhile. It did for me. Most of the other antidepressants (in the SSRI family - which Wellbutrin is not - but Zoloft, Effexor, Paxil, ect) have been known to interefere with sex (again, they certainly did in my case). Mainly they make it very difficult to orgasm, although they can affect libido as well.
Whether his lack of sex is contributing to depression (which is why a Dr would prescribe) is hard to say, but I also doubt he would be put on it for just that reason. He probably has other contributing factors, and if the doctor is worth his salt he would have tested him first. Also, it shouldn't "bring him down", it's supposed to be a mood elevator. If it isn't doing that then it's probably not the right med for him.
Not that it's related to what you asked, but Wellbutrin is also the "anti-smoking" drug, Zyban. It's just a weaker dose.
DVDBear