Bms at School

there was a big long string about this subject several months ago and one person said that they were like this as a child and that it was because they 'felt the urge' to go WAY before it actually came out. So they use to have to sit there for like 20 minutes waiting. I never followed back up but you may want to do a search to see if anyone had any good information re: if is a way to help them learn when it's closer to actually coming so it's a more normal 3-5min. trip.

I think it will all change around again on it's own.
My son (also 8) only now (3rd grade) feels "comfortable enough" to poo at school! He used to hold it no mater what. So...it's a good thing your son will "go" at school.
Maybe some high fiber stuff over the weekend?

While you say he is not constipated, you could add things to his diet to make it a bit easier to go. Make sure he is getting enough fruit & fiber.

What is he eating for breakfast? Will a glass of juice help? Is he going after a big meal like lunch? Can you give him a bigger breakfast and allow time to go at home?

What about a prune at night or a box of snack raisons at night, so he is ready to go in the morning time?

I don't really have any advice I just had to say how surprised I am that he tells you about his BMs at school!
And you know he may just be using that as an excuse. Little boys generally deplore handwriting ;)