My nephew stopped cold turkey and started suffering with terrible stomach problems. He couldn’t keep any food down. He lost a ton of weight before he found a psychologist who had suffered the same thing. He had been on ritalin since he was a little kid, then when he was in high school the doctors added zoloft because they said that he was very stressed. When he graduated from college, he decided, no more and just stopped.
I can relate to what he is going through, tell him to talk to his Dr, he can ween him off rather than go through what I have/am.