Hi. Excise, particularly intense exercise can cause people to go high.
You need to review what he is eating and drinking and the carbs in them. Is this enough to explain why he was 300? Or could he be high also from the exercise.
He may need to bolus for both the exercise he is doing and whatever he is eating during the exercise or alternatively, he may need to increase his basal rate during exercise, and reduce again after (as well as bolus for the carb he eats).
Do you know how much one unit of insulin brings him down? If he is 300 and want to be 120 you need to bolus him for 180 blood sugar. You will need to experiment with the timing for the insulin. Maybe 1/2 when he starts training. 1/2 test and see if he is high in the middle of training and bolus the rest. Monitor closely after the exercise (eat a protein /fat snack after) as sometimes people will go low after exercise (even if he went high during)..
it is very individual how he will respond. So you guys need to monitor, keep track, and make adjustments.
Hope this is helpful for you.
Water, not too much. How long are the training runs? He should be properly hydrated before he starts. Then a guideline for endurance sport is 500 ml - 800 mL / hr. If it is cold, aim for 500 / hr, also depends what he is comfortable with. But not more than 800 mL. He should also drink after if he is thirsty.
I do basal-bolus only, no pump. But you could try putting the pump into a hydration unit holder.;
During the event (at least for now until you understand how his blood sugar reacts), testing every 30 minutes. later less frequent. Test after the event every 30 minutes to 1 hour until you are sure he is stable. Test before bedtime to make sure he is on target.
Sorry, more: I reread your question again:
He may need to limit his carbs during the training. Particularly if the session is short (or at least no carbs while you test and see what the training is doing to his blood sugar).