Father claiming to act for son

I have a former tenant who owes two months rent, plus damages. I sent a pre-claim letter (Notice before action) to two addresses I have for him and received a reply from his father claiming the son is suffering from clinical depression. Given the prescribed medications togther with scales and illegal products found on check out, I can well believe it.

I do not believe that the father can legally act for his son without either the son's written consent or a medical certificate. Before proceeding to court to recover nearly £4000 I would like advice on whether to even discuss this with the father, especially now that we have GDPR.