Obviously you have to tweak this based on his activity level and personal trends, but when I play soccer I have about 50 grams of carbs with half of my normal bolus dose for dinner. Someone correct me if I’m wrong, but my understanding is that when you engage in intense exercise you deplete glycogen stores in the liver and muscles, which your body then tries to replenish afterward as you recover. In order to replenish, the body basically pulls glucose out of the blood. It’s not always easy, but you have to estimate how much glucose needs to be replenished and over what time period, and try to match that. For me, with intense exercise, I do this as a factor of time. I typically need 1 gram of carb for 1.5 minutes exercised. I spread this out a bit too, to try to better match the rate at which my blood sugar is dropping.
Note - after exercise a body becomes more sensitive to insulin. Also, if you have hardly any active insulin in the body, blood sugar might not come down on its own despite the exercise. Therefore, If I just ate the carbs I needed to replenish, but didn’t take any insulin, then my blood sugar won’t come down. However, a meal with half my normal insulin dose seems to do the trick.
Good luck. It takes a little bit of trial and error, but it’s possible. Look on the bright side - it’s a rare chance for him to have some forbidden foods without high blood sugar.