Does your son accept that he contracted to take the course and pay the money?
Depression would not necessarily indicate a lack of legal capacity to enter into a contract although it may do. In order to defend this, assuming they pursue payment which they may not, your son would need medical evidence to show that he did not have the mental capacity to understand what he was doing when entering into the contract. From what you say he was not in a fit me to state to take the decision so a visit to his doctor would probably solve this by getting a letter to say he was not of legal capacity at the time. I hope that helps. Please leave a positive rating so that I am credited for my time.