When joking, you may sometimes touch the other person's sensitive points, especially when it comes to personal identity and family relations. In your case, calling your boyfriend "son" may make him feel uncomfortable or hurt, because it may make him feel that you don't respect him or his independence.

Although you think this is a joke, everyone's acceptance and sensitivity to different words are different. So you need to consider his feelings, apologize to him, and let him know that you understand his reaction and are willing to respect his feelings. You can try to communicate with him, understand his thoughts and feelings, and find a mutually acceptable solution.

In addition, in future communication, you need to pay more attention to your words and phrases to ensure that you don't use any words that may make others feel uncomfortable or hurt. If you are not sure whether a joke is appropriate, you'd better not use it.