I am looking to identify a series of fantasy books for young readers, which I was aware of in the mid 1990ies.
One event I remember was that the main character, a pre-teen boy, would travel from our world into this fantasy realm and then return. After some time in our world he would travel to the fantasy world again. When he returns, he finds out that time has passed much faster in the fantasy realm, and a girl he was friends with the first time is now at least a teenager, and no longer interested in him. I clearly remember his frustration.
Each of the boy’s visits may have been the content of an individual book. I do not know how many there would be in total. The main story of each book would be some adventure that the boy gets involved in (none of which I remember).
The access point / gate to this fantasy realm may be in the boy’s grandparent’s garden. And the time between visits may be motivated by him only being at his grandparent’s during the summer holidays.