He has gone in as requested and it is making him worse. Therefore he has attempted to accommodate the managers wishes but it is now affecting his physical well being. He should explain this to ATOS when they call and advise them that he will be returning to his own Doctor to explain the situation and will not go back to work, even on light duties, until his own Doctor feels he is good and ready.
ATOS can request that he gives their own "specialist" a visit but ultimately he can follow his own Doctors advice. He could also point out that the whole situation is making him depressed and that the more pressure that is being put upon him from his manager to go on light duties, the more depressed he is feeling. Which I expect is very much true!
I'm sure if you backed him up Davey it would be enough for him to stand his ground. Sometimes it just takes a little support for someone to feel strong enough to stand up for themselves!
It also might be a good idea for him to rejoin the union at some time in the near future.