Advice from Two Different Generations. Research some of your local counseling agencies and suggest to your husband that you visit one of these, either together or separately. It sounds like your husband has such feelings as hopelessness and of being trapped in his situation, yet you comment that he still shows brief signs of wanting to regain the wellness of your marriage - I'd advise you to help him take the steps to do that by continuing to give him support and by offering new avenues for alleviating his depression. The obvious answer is to get your husband to a doctor. His depression could be a result of a physical condition.
Depression is nothing to fool with and can be managed with proper medical care.
If a physical problem is not the cause, then perhaps a visit to a psychologist is in order.