Thanks for your question.
This is a common problem for active kids and teens with type 1 diabetes. Each person with diabetes has a very individual response to exercise and there is no standard answer as to how to manage this. Some additional things to think about include:
Insulin adjustment - we don't know your son's insulin regimen, but many people find that they need to do a lot of insulin fine tuning before getting it right. This can also change over time, with factors like growth and development, puberty, different types of activities and pre-match anxiety all affecting BGLs. Many people find that they need to adjust more than one type of insulin, in your son's case if he's playing sport in the morning you may need to make adjustments to both his long acting and his short acting insulin. Sometimes you may need to reduce the usual insulin dose by up to 30%. Your diabetes team can guide you on how to go about this.
Remember there is also a risk of delayed hypos 12-16 hours following exercise, so Friday evening sport may also require changes to your son's long acting insulin that evening. It's a good idea to talk to your diabetes team, asking them whether making these adjustments will help your son's situation.
Testing - to understand your son's specific response to exercise it's a good idea to do extra BGL testing before, during and after sport. This will make it easier for you to find a pattern and fine tune insulin adjustments. Many 15 year olds may be reluctant to do these extra tests, however in the long run, if it helps prevent hypos during the game, or any adverse effect on performance it's worth it.
Food - It sounds like you're doing a great job ensuring your son has plenty of extra carbohydrate foods for exercise. You don't say when he's having the extra food, it's important to ensure he has extra before, during and after the game. Both the amount and type of carbohydrates can make a difference. If you find he's running out of fuel, slow acting or low glycemic index carbohydrates before the game such as raisin toast, canned fruit, yoghurt or even a mini chocolate bar may help prevent his BGLs dropping low. During the game, it's a good idea to make sure he has extra quick acting or high glycemic index carbohydrates every 30 minutes or so. A food diary that allows you to work out how much carbohydrate he has eaten and when, can also help you to make appropriate food adjustments for sport.
Unfortunately we can't provide prescriptive advice on this website, it's important that you talk to your diabetes team about managing diabetes and sport. Go armed to your next clinic visit with lots of BGL results, a food diary and some ideas about insulin adjustment to discuss with the team. A great book that may also be of help is "Caring for Diabetes in Children and Adolescents - A Parents Manual" available from Diabetes Australia-NSW and the Children's Hospital at Westmead.