You need a 504 plan for him so that ALL educators in the building are aware of his issue and able to help him. This is not something a six-year old can self-advocate for, IMO. Get in touch with the child study team at school and set up a meeting. Your doctor will probably have to provide some sort of documentation. In the meantime, tell his teacher. Get a doctor's note. But I'd definitely recommend the 504 for going forward. You can always not renew it once his physical development catches up.
As a teacher, I get all sorts of 504 plans, and they just let me know when I have to provide leeway for students because of medical reasons--frequent bathroom passes, different seating arrangement, snacks for diabetics, etc.
