Firstly, I think it's great he's taken this step, and a brave step into the unknown for him. I hope it's the beginning of a journey of healing for him.
On the matter you ask about, I couldn't possibly advise. I'm not a medical doctor, and am not qualified. It sounds like a complicated situation that needs professional advice. I'd suggest getting in touch with a fasting centre for initial advice and guidance, with a view towards doing the first fast supervised by a professional in some way, if possible.
I suspect a medical doctor would advise resuming food immediately.