Maybe try shoving some fingers in his ass and if that works, hire a transvestite prostitute to hide in the closet and surprise buttsecks him and see if that gets him off.
In all seriousness though, does he watch a lot of porn and masturbate a lot? The mind and the penis can be desensitized by overstimulation of the mind and desensitizing the penis itself. Other possibilities are that he has some fetish, a fetish by it's true definition is something external outside of normal sexual contact needed for sexual satisfaction or it could be certain sexual acts or maybe some sort of role play could help him. Could also be a sexual preferance thing. Ask him where his mind was the other times you brought him to climax.
Is it your technique? If you haven't had problems with other men then it's not this. Just trying to cover all bases.