SOLVED: YA sci fi, a kid gains mindreading powers after encountering radioactive swamp water [s]

[Anne]

The kid's grandma (?) encourages him to use the swamp water powers to fight malicious people of some kind, and a girl/girlfriend is also involved. I remember the kid seeing his grandma's mind as a busy knitting factory or something, but later when she's been mentally compromised the factory is all empty and cobwebby.
I read this around 2005 but it was probably from the 90s or earlier, and I remember the cover looking sort of B-grade.

[Andy]

William Sleator's book Others See Us Others See Us

[Anne]

Perfect that's it, thanks

[Andy]

Glad to help!