Hi JC
What you do next depends on your son's current health and a bit on the outcome you are expecting.
If your son's health has "improved," ask yourself if he still fulfils the PIP criteria of his current award. If he doesn't, you should inform the DWP as soon as possible. Depending on his current award, they would review it, and there's a good chance they will downgrade or remove it. The system can't take account of his condition deteriorating at any time.
However, you don't have to tell the DWP if he goes to work - you only have to tell them if his symptoms have improved. So if you think his mental health difficulties remain the same - there is nothing to report.
As his appointee - you are the one who knows that. If you need to report a change - you ring and say so.
If his award is downgraded, he can always apply for a change of circumstances in the future if his health deteriorates. If his award is removed, a new claim could also be made when necessary.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems