Guy's have a hard time getting erections under pressure, thats why they aren't all cut out to be porn stars (as much as they'd love to be lol). He know's your waiting for it to get erect, hes waiting for it to get erect and since hes focusing so much on that , he can't relax and enjoy the sensations enough to actually make it work.
If you are not "allowed" to give him foreplay, wow. Its no wonder he has trouble getting erections. Sex therepy ASAP. He can't will his penis hard, I don't think. I think that there needs to be some kind of tactile stimulation going on. If he wont let you touch it, maybe work on visual stimulation. Pop in a movie that is not an overt porn , that would be too obvious, but a regular movie that happens to have some hot sex scenes... I think unfaithful might be a good one
Maybe an impromptu lap dance or striptease if he is okay with something like that, while he foreplays himself? You are really up against a brick wall there cassie, it sounds like he's being difficult on purpose. Has he seen a medical doctor? Does he consume a lot of alcohol or on any antidepressents? If he is physically healthy, marriage counseling is really in order.