If he was very recently diagnosed then his blood sugars will still be high, insulin doses are started conservatively as it would be dangerous to drop his blood sugars back down to normal too quickly. This is bound to affect concentration, not to mention just the shock of the diagnosis and trying to take it all in. Speak to the school, he should have a care plan in place which details what needs to be done during the day to manage his diabetes, what to do in an emergency and so on. The diabetes specialist nurse should be able to help with creating that.
Also, he should be entitled to breaks during exams if necessary, for example if he goes low and has to deal with it and needs time to recover they will stop the clock, let him do whatever necessary and then continue when he feels better. Again the nurse/hospital team should be able to provide a letter to this effect. Also that he needs to have blood testing equipment/CGM reader and hypo treatments in the exam with him.
Other than that all I can suggest is just keep as close an eye on his blood sugars as he can and learn how to react to problems. It’s a steep learning curve at first but does get easier, so hopefully by the time his final GCSEs come round he’ll be in a better position to deal with them.