You guy is not only scared, he's scared 'unstiff. ' Short of using wooden splints and duct tape, there is no sure fire way to help him keep his erection.
The point is that this man must figure out and resolve why condoms get him down. I can give you some ideas on how to point him in the right direction. First you need to understand how such a problem usually develops. Putting on a condom is a technical operation and can cut a man's arousal level. When his arousal flagged, he lost his erection.
The next time around he worried that he would fail again. The worry alone caused him to secrete stress chemicals, and these chemicals prevent a man from achieving an erection. So what can we do now?
One thing to do is to help him lessen his performance anxiety. You can tell him that if he loses his erection, it's all right with you. He can go down on you instead. When he feels that you aren't putting pressure on him, he will relax, which will help him to achieve an erection. You might also try a little exercise in which you tell him up front that intercourse in not on the program tonight. Put a condom on him and masturbate him. Tell him that no matter how hard he becomes, he isn't allowed to penetrate you, again removing the pressure to perform.
I am hoping that with the pressure off of him he will be able to achieve an erection while wearing the condom. If this happens, we will have successful broken his negative association between condoms and limp penises. If you are still stuck, then go together to see a sex therapist. Let me know what happens.