It is very likely a bigger irritation for him, than for you. And it is not you. And very likely it is not him. I think you have now pretty much figured out his penis has a head of itself, deciding what to do when. It is important for him to see his doctor to make sure itis not a sign of another problem. Even if he does not want any Cialis, Viagra or Levitra, his doctor still needs to make sure his cholesterol, heart and blood flow is good.
It is very often caused by anxiety and stress when there are no obvious medical conditions.
What is the quality of his original erection like?
A few suggestions:
1) Take the pressure off him for penetrative sex. When he loses his erection, don't let him go to masturbation. I am sure he feels humiliated when he masturbate to get hard only to lose it again. And i am convinced that it is not what you want. Continue with foreplay. Give him a massage to relax him, give him a blow job or hand job, or let him go down on you or finger you. Take his mind off the immediate panic and disappointment. Just ignore it like there is nothing wrong and continue with other things. It sounds like when he is not thinking about the problem, his erection will come back.
2) just go to bed and wake up in a few hours time when he had some time to rest and relax. If you are up forint, wake him up with a blow job. I am yet to meet the guy that will not think you are the best thing that happened to him if you wake him up like that. Try again then when his mind and body is more relaxed.
3) swap roles. If you are normally the passive partner, take the lead and let him lie back. Try different levels of aggressiveness, gentle and loving, aggressive and very aggressive and domineering. See if any of these changes make a difference.
Lastly, have you tried to track a pattern of when he is good and when he does not perform well?
Best of luck, and remember that he is even more difficult for him. A man's ego is closely tied to his performance and ability to please you. You can be angry that he does not want to seek help, but not because this is happening. And you need to make that clear to him.