I read this book at school in the early 1990's. I seem to remember the title being "Summer of [blank]" "Spring of [blank]" etc.
It's centred on a village that has a stone circle. I vaguely remember the boy having an archaeologist father, and travelling through a secret passage in a Stonehenge-like stone circle and discovering another world underneath.
The characters in each book were recurring in each book of the series, but on different points of their time line.
A snake definitely became a major plot point in the later ones (possibly winter and autumn?) I would really appreciate any one who could even say they've read the books! This has been bothering me for years!