Yes ^^ contact the court and tell them he no longer lives with you and give them the correct address. Or just pass the letter on to him and get him to sort it out, it is not your problem he is 18 and an adult.
If bailiffs do turn up just explain to them that the person concerned does not live there. This happened to me, my daughters ex partner ran up a council tax bill and gave my address. The bailiffs turned up and I explained the situation, I showed them proof of my ID and a rent statement to prove that I was the registered tenant and they went away. I told them I did not know his current address, which was the truth.