Indeed you use positive reinforcement but not the way you expected. It sounds like your husband positively reinforced the parrot for biting him in the cage. Whatever it is that he did increased the likelihood of the biting in cage behavior, therefore it is reinforcement. I'm not certain if he is leaving the bird alone after a bite or what he does at that stage (which would really determine if it is positive or negative reinforcement but this is of no consequence as in fact he is reinforcing the aggression).
You may need to fill us in on more details of exactly what is going on step by step. But with what I have heard, I think a complete do over is necessary. The good news is that it's not too late and if you fix this now, there shouldn't be any long term ramifications.
I would suggest (for now) that since you can successfully remove the parrot from the cage and your husband can't, that you be the one to remove it until this step is completed. You remove the parrot from the cage and put it on a consistent training spot (preferably a
training perch). This spot will become a cue that everything to come is good. To condition the spot, for the first few days you can have the parrot do some tricks here. Even if you haven't taught it any behaviors at all, you can find some simple thing to teach like target just for the sake of letting the bird earn treats.
Before the parrot can get too established in the spot, but once it develops a reasonably positive association with it, terminate any further provision of treats, training, or other social interaction between yourself and the bird. You can only act as the mediator by taking it in/out of the cage but you need to let your husband get the credit for all the positive stuff. This isn't necessarily going to happen on the first day, but with time it will. Your husband should get a bit of a head start since the bird will have already been conditioned to the training spot and the trick behavior (like target). He can practice that behavior and use it as an excuse to offer treats. It doesn't matter that the bird is already tame and already knows the trick. The point is that this is an opportunity for the parrot to learn proper behavior with your husband. Simply offering treats to be nice doesn't do it either. There must be a specific positively reinforced behavior because this becomes an alternative to behavior to biting. For a few days he should work on this, keeping sessions only to as long as they are fun and the parrot's attention is maintained.
For the few days of the step above, your husband should not force the bird to do ANYTHING AT ALL. He should not ask it to step up, not touch, not pet. He should only practice positively reinforced hands off behaviors. This is to erase the damage done. This is to help the bird forget about being touched or forced in ways it did not want before. The parrot is to relearn that your husband is not going to force it or do anything unwanted again. Also this teaches that your husband is not scared of biting (bringing biting to extinction) because hey didn't we agree to avoid any chances for biting by not forcing? The idea here is to set the parrot up for success and make biting not work by not even creating situations where biting could work or even happen in the first place.
Only after consistent success with the hands off training for several days straight, should the taming process be initiated. Once again, don't forget that in this process all good things (short of coming out of the cage) should be coming from your husband. Don't worry, the parrot will still like you and you can always earn his trust back easily. At this point, slow/steady training should be done to reintroduce touch in a non-forced positive way. This would be first holding the hand a foot away and rewarding. Then 10 inches, 8, etc. You may have to stand at a distance and coach your husband if he is not adapted at reading the parrot. But as soon as you notice some discomfort from the parrot, that is the time to say stop. He should hold his hand at that distance for a few seconds (progressively increasing the duration) and then take it away (negative reinforcement) and give a reward (positive reinforcement). Eventually he'll be able to touch the parrot this way. You are more familiar so you should be able to coach him which places are most acceptable to touch. If you don't know a specific, I would
suggest the beak and neck as good places to start. The good news is that after doing this enough times, the touching can in itself become positively reinforcing since they like beak rubs and head scratches. So eventually additional rewards will not be needed.
Only after this point can he begin step up training. Of course the
target training step up is the best way to do it but it could just be request to step up as you do as well. These should be consistently rewarded at first and then dwindle it to a variable ratio. Once this is all comfortable in the training area, the same exercises should be repeated all around and then eventually in the cage. The pace goes much quicker where it might take a week to get the first touches but the only a few sessions or even instances before it can be done on other perches toward the end.
Since the parrot is already tame, these should be quite easy to do. However, even if it is possible to do this in under a week, I still recommend drawing it out for at least one week just to give the parrot time to forget the old habits and develop the new. It may take longer though. Once your husband develops a positive approach to his behavior, I think he will not only be able to develop a safe relationship, but also maintain it in the long run. Once your husband/parrot relationship is improved, you can return yourself to the interaction as well. Just be careful not to be upstaging your husband in the reinforcement side because it seems the parrot prefers you anyway, so leave the bonus treats for your husband to use.
PS I think clipping has a lot to do with the trouble you are having. If the parrot weren't clipped it would just fly away from bill at its first displeasure with him and bill would be unable to continue trying to force it. However, since it is clipped and stuck with the repeated attempts to be forced to do what it does not want to do, it is left with no method besides biting. Clipping breeds aggression.