As others have discussed, having sex with your husband, even though he uses contraception against your wishes, is allowed. But you can’t cooperate in the contraception, so a hand job is definitely a no. But having sex, while he is wearing a condom, is OK, assuming you haven’t helped him put it on, encouraged him to do so, bought it for him, etc. He needs to know your objection, but it isn’t something you have to bring up every time (which would be a major mood-killer).
I also find it difficult to believe that his “erection problem” is caused by your unwillingness to give him a hand job. That’s such baloney I don’t think I could do anything but laugh in his face if a man said that to me. It sounds like something a boyfriend would say when you’re 15! If an “erection problem” actually existed that is caused by lack of maturbation, then he could just as easily solve it by engaging in the marital act with you on a day when you are not fertile. (Or preserve your dignity by taking care of his own “erection problem”.)
Yes, I think baloney is the right word here. What exactly is an “erection problem”?

Men get unwanted erections frequently, especially adolescents and young adults. We can get them in our sleep, as we are waking up, in the shower, etc. It’s just blood flow and erections pass quite naturally without the need for ejaculation.
If, however, arousal is indulged in without climax, it can get uncofmortable for men. The easy solution to this is not to indulge in arousal when you’re not going to complete it via intercourse. If you’re not going to have intercourse, don’t engage in foreplay.
Either way, I dont think there’s such a thing as an “erection problem”.