Therapy is still important, and I hope that he will change his mind and give it a try. Yes, psychosis can be related to severe depression. He might not become psychotic again, but it will be much less likely, and he will also have much better chances of managing his depression, if he gets therapy and understands where these problems were coming from, and learns skills to recognize and manage the symptoms. Medication helps to manage symptoms too, but it is not a cure. Therapy can treat the roots of the problem.

The other side of it, though, is that it needs to be his choice to go to therapy. It won't work unless he is willing to do the work. Nobody can change somebody else. Since you have been affected by what happened as well, and you are still concerned and probably experiencing some distress, you might consider getting therapy for yourself. It won't change your husband, unless maybe he sees you benefitting and decides to give it a try, but it will help you to cope and to be able to decide what to do.