Jo
The DWP cannot require a claimant to take medication or treatment for their conditions, in principle, they can assume it if they believe that the claimant's reasons for not doing so are unreasonable but I have never heard of a case of them doing this, not that I think they would in your son's case as he has a good reason for not taking the medication.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems