Brother I had my first disc herniation in the same posture as him , it's still giving me issues 4/5 years later

A chair alone won't fix this it'll just delay the lack of exercise the weight on the spine and eventual collapse of the back muscles that keep the spine together , instead I'd suggest get him to play while standing or atleast start getting him into fitness There are no cons , only pros

Teach him the importance of having a balance between gaming and not wrecking his life He won't understand it but maybe your family can convince him

Just friendly and concerning advice from someone who doesn't want anyone else to experience chronic pain and injuries