My 13 year old has been measuring himself against the door frame since his uncle's joke

At a family lunch over the Dasara holidays, my brother-in-law joked that my son 'will be the shortest one in the class photo again'. Everyone laughed, including my son. He's 13 and hasn't had his growth spurt yet. Most of his friends have. Since then he has been standing against the door frame every morning and making a pencil mark. He hasn't said a word about the joke, and when I asked casually why he was measuring, he said 'just like that'. I want to bring it up without making him explain why it hurt, because I think having to explain it would be worse than the joke. How have others started that conversation with their child? The conversation with the relative I'll deal with separately.