Just because the spirit is willing doesn’t mean the flesh is up to it.
Boner problems come in two flavors: emotional and medical.
Start with medical. The penis is the body’s early alert system: if it’s not working, something else is probably in trouble too. He should talk to his doctor about his cholesterol, and get a check-up. If all else fails, Viagra, Cialis, we all know this drill.
But don’t rule out emotional problems, either. Even a loving, caring guy can have roiling waters under the surface that he’s not talking about. Don’t try to be his therapist, but if you think he needs one, encourage him to visit.