Kids book series about a boy who goes into another dimension? And trys to get back and keeps getting so close but cant't?

[Mackenzie]

I probably read these books between 2008 - 2015. I'm looking for a book series where a boy either mum or dad go to get milk (I think) and the boy ends up floating down a river and going into another dimension and he goes on lots of adventure while trying to get back to his home. I remember reading about him going into a mountains and finding a yeti maybe and going to a marsh and every time he gets close to getting home then something happens and he cant. It'a a long series and each book had a solid colour on the cover and each book had a different colour I think. I also think the book might have been written like the boy was using as a sort of journal/diary, not sure though. I think the boy in the series did come home at one point in the series but had to go back? I also think the last book I read from this series many years ago ended with something about a train, I'm not sure though. Any help would be greatly appreciated!!