I am a long distancer runner training for a marathon. I have lived with type 1 diabetes for thirteen years. I use the Bernstein dietary regimen and "exercise technique", which consists of eating small amounts of dextrose while running to compensate for the hypoglycemic effect of exercise. The aim is to keep blood sugar normal at all times. It works. Google "Bernstein diabetes" for the book.
I am not a physician, but a volunteer and the mom of a type 1 diabetic.
I would suggest you call your son's endocrinologist or CDE (certified diabetic educator). I would go over your sons training schedule, school schedule and meets. They would be the best to advise you as to how much your son should eat as a snack before hand and how to regulate his insulin for meets. Exercise and stress can lower Blood sugars so your son might require less insulin and more carbs. I would also discuss with your sons coach on what he needs to do for his diabetes, and I would also make sure that he knows how to use a glucagon if needed.
Since your son is active in sports I would also make sure that what the doctor states is put in to your sons 504 school plan so that they accommodate all his needs not just academically , but for sports also.