If the weight loss program focused only on weight loss, I’m not surprised that your son wasn’t successful. He is probably too depressed to have the discipline to do a weight loss regime.
My guess is that all of his health problems are inter-related. What they have in common is the depression. For that reason, I would want to focus on that first. A depressed person is simply not able to sustain the motivation needed to manage a weight loss program. The depression may also be at the root of his lack of friends and a social life. (Overweight people do have friends and lovers but not if they are chronically depressed.)
Ask your son to talk to his doctor about how diabetes and sleep apnea contribute to the depression. If he isn’t using a C-PAP or Bi-PAP machine, encourage him to talk to his doctor about whether it would be helpful. Many of my clients who gave the machines an honest try felt much better within weeks.
Further, if you haven’t already, encourage him to see a therapist who can help him deal with the depression directly — and who can be a weekly (or more) support in his life. Suggest to your son that you will be able to step back and give him more space if you know he is seeing a therapist who has skills you don’t have to help him be less depressed.
Meanwhile, as difficult as it is, it is time to take a step back. As you said, your life and happiness are too much entangled with his. That not only gives you stress, but it also puts pressure on him. You may be inadvertently adding to his depression if he is feeling responsible for your distress. Further, you certainly don’t mean to, but by primarily talking about your concerns when you speak to him, you may be reinforcing the depression.
By all means, ask him to call you with good news but suggest that, for awhile at least, you will take a break from always talking about his health. Then take steps so you can have positive things about your life to report to him when you are talking to each other. Make a list of the things you used to enjoy before you got so wrapped up in your worry about your son. Then make every effort to reclaim your involvement in some of those activities.
You certainly have legitimate reasons to be very worried about your son. But sharing the worry with him hasn’t been effective. Why not talk these suggestions over with your son and see what he thinks might be more helpful?
I wish you well.