There is plenty that can be done. I wouldn’t say no stress reduction is possible, because a huge component of stress is mental. Maybe he can’t change the job, but he can change his own stress reactions and behaviors. For example, if you can get him to eat regularly during the work week (maybe pack some things that are very easy to eat), that would be something. However, you can very rarely change or save another person. He will have to want it.
Also, I would check out the side effects of metformin, as I mentioned above. Here’s a drug.com link for starters http://www.drugs.com/sfx/metformin-side-effects.html Just looking quickly, it looks like sometimes a B12 deficiency can develop. B12 deficiency will make you exhausted and lethargic, so at the very least I would recommend he have a blood test to check. Sounds like stomach issues with the drug are common. too.