If he's that worried about it, why is he going at all? He could say he's got other plans that day or just plead sick, if he really wanted to do himself a favour. It doesn't sound like he's strong enough to deal with such a stressful, triggering situation yet. There's no shame in deciding not to put himself through that. And it's not your responsibility to make sure he doesn't relapse either.
Addiction recovery treatment is really important if someone is to learn how to cope with trigger situations like that. That's how relapses happen and they can be avoided and/or pulled out of quickly if he learns how to treat situations like that as an opportunity to learn new coping skills. He can learn that through most addiction treatment centres that take a problem-solving approach to it. A treatment centre could also help him figure out what meds would help with the mental health aspects too, since that can complicate things.
I would also agree that you need to get some support for yourself. If your husband is willing to go to a treatment centre, they often have programs for family members too. But even if he isn't willing to go, check into it yourself, either a structured program or Al-anon. There are many books out there too, that can help you understand how you react and interact with his addiction and how it becomes a way to avoid dealing with your own stuff. It might not seem like it right now but it does.