As someone who IS him(not literally) without the sexual dysfunction I think he should seek some mental help. He does sound like an emotionally repressed person and talking to someone may help.
The fact that he's able to have a girlfriend at all is amazing to me so it's possible she sought him and not the other way around. Regardless, it's more likely to end with more whimper than bang unless he gets angry, and I mean truly angry. Then all bets are off.
Signed, guy described to a 'T" but actually got help.