My boyfriend struggled with this too! He'd go soft in me and i'd have to blow him multiple times just to get him up only for him to go soft again. I got really insecure and we talked about it and he told me about his death grip thing. He would jack off like twice a day. His penis lost lots of nerve endings and feeling.

So I started setting goals for him. For example, if he jacked off no more than twice this week we'd get to use blindfolds when we have sex or try anal or watch porn together etc. Each week he achieved his goal, he received praise and the next week we'd make the goal a little more difficult to achieve. He's really put his mind to it and it's been working for months now.

I also blocked some porn sites on his phone so he wouldn't have such a strong urge to masturbate so often. Little by little I've been giving him back his materials to jerk off.

I also try not to give him handjobs so I can train his dick to respond to other sensations. You just have to believe in him and support him and he will get there. Good luck! He will surely thank you!