Can you get him to a AA meeting either in person or online?
It is unfair and unreasonable of him to expect you to shoulder all this burden yourself. He has to take steps to deal with the addiction outside of just going cold turkey (which generally seems to lead to relapse). Has he said why he doesn’t want to confide it has GP?
Have you looked up drug and alcohol services in your area? They are usually separate to the MH services. You might find they are more help.