Sorry for what you’re going through! I have a 9 year old daughter so I’m not in the same boat but wanted to share a few thoughts.
If you are paying for the phone then cut off his service or cut off the games feature of the phone. If he doesn’t have the phone to distract him then he’ll find some other way to spend his time, hopefully in a more productive way,
Next, can you find out what is going wrong in the training program? Is he not really interested? Is he too distracted to concentrate on the material? Why is he about to fail? Is it fear? Is it lack of motivation? Is he unable to concentrate on his studies?
Can you afford to send him to a therapist who has a lot of experience with add who can help him evaluate his life and come up with a realistic plan of action?
I know he’s 22 so there’s only so much you can control but I would also look into other lifestyle factors like diet and exercise. Can he join a gym? Can he join a hiking club? He needs to be a part of something positive and then try and build from there!
Wishing you the best of luck!