Perhaps it's not so much that he's "not interested in sex"--as you did say "he does have sexual desires"--but that the part which is "supposed to react to sexiness and allure" just doesn't feel free and secure enough to do so. If he grew up in an ultra-conservative, overly controlling, sexually oppressive, and/or abusive family environment, he may instinctively feel as if sexual enjoyment is a thing to be loathed instead of loved. Going against the grain of being "raised to NEVER make the first move" can be tremendously difficult, and fighting to allow certain enjoyments that were previously forbidden can create great discomfort and pressure that could easily result in loss of erection and lack of activity. (If it does stem back to his upbringing, a counselor may be a good idea.....or buying handcuffs/whip for HIM to use on YOU--lightly, of course-- to work on and push through his issues of dominance. Just a thought )