A book about a chubby boy who spends summer vacation at his aunt and uncle

Hello, I am trying to remember the title a children book series about a chubby boy who visits a town in which there is a lot of strange things happening. He stays at his aunt's and uncle's house. I remember one particular scene from the first book in which they were running from some witch in a car and then the uncle said that they should cross the river because the witch wont be able to follow them. Also I remember that in one of the later books in the series the boy is trying to prevent some old guy from summoning some Cthulhu like creature. Please help me find the name of the series.