It’s a tough spot for you to be in. Shaving cream could help. It’s a good idea that you don’t have him shave too often, to avoid unnecessary confrontation. Also having a little more growth might make it easier for him to see what he’s missed. If he’s okay with you checking for missed spots most of the time, maybe it would help if you had him feel for the missed spots instead of pointing them out. But, if he gets upset about it, I would drop it right away. I know you don’t want him to look scruffy, but if it’s a point of pride for him, try to accept a little scruff. Then, since it didn’t turn into a stressor for both of you, you have a better chance of circling back to it. Maybe you offer him the razor again sooner than you normally would. (Would he accept a good shave once in a while from a barber–another option?)
Eventually a time will come that he needs someone to do it for him, but sounds like he’s not ready for that yet.
When you have a chance, you may wish to check out some tips I like from Teepa Snow. She recommends a method called “hand-under-hand,” which is a way to help a person to perform tasks and let them feel they still have control. It sounds like your husband wouldn’t be ready for this with shaving yet. However, tuck the information away for later if it isn’t helpful yet.
Good luck to you, and be sure that you take good care of yourself, as well as of your husband.