Here in Melbourne most children go to kinder for 11+ hours a week and there is talk that these hours will increase. They are tired the first bit but usually settle in well and get used to the hours. When they are at school the hours are alot longer as it's 5 days.
Two days of 2.5 hours is not alot for them to cope with.
I think the thing you need to consider is how he will cope with the two different styles of learning and structure.
Is there any way you can try the extra day for a term to see how he goes?