Telling People

My 8-year-old son looks like a pretty average kid, but he has some motor and language delays that affect his day-to-day functioning and make him act more like a five-year-old. When he starts new activities - like scouts or karate - I'm never sure if I sure approach the teachers and tell them about him or just let them figure him out on their own. I hate to label him as "delayed" in their minds - but on the other hand, it's going to become obvious to them early on that he's different, and it might be helpful for them to know what they can expect from him. Any thoughts on this?