It feels to me like you man shows signs of depression. The drastic changes in his behavior and the weight gain indicate that he is not coping.
I would suggest that you encourage him to get professional help. This kind of depression is triggered by a change in circumstances, and even more by how we view those circumstances. He obviously does not see any benefits from the situation at work, and got stuck in the downside of the job.
Once he understands what has changed at work and that there is in fact some benefit for him, the depression will lift. Is the change due to a challenge that he feels he cannot cope with when in fact he is doing better than he thinks?
Has he outgrown the job and it is simply time for him to move on? Is his stress due to being frustrated at where he is when he should in fact have left already? Often the behavior of people at work indicates that we have overstayed our welcome and we cling to a job purely because we feel we are not ready for a change. The behavior of our colleagues then indicate in a not so subtle way that we need to either go or be thrown out, so that we can face the next challenge – which is invariably an improvement on what we have.