It’s really tough isn’t it. Have you tried asking him about his local Alcohol service, as they are all still working and most of them are offering virtual groups including AA and NA fellowships.
Sadly, like anyone addicted they have to want help. The trouble with alcohol is it’s a depressant so his low mood will only exacerbate his intake. I wonder if his key worker from the previous rehab would reach out? Worth a shot, to remind him what he can achieve?
All you can do is encourage him to seek support, and look after yourself.
He probably needs to tell his GP also for a Review of his health.
Best of luck ????