this makes no sense, there are IP addresses used by thousands of people like in universities or companies.
the only explanation is a cookie of a google service (can also be YouTube, Picasa or whatever) left in your browser.
check your cookies! and i recommend to delete them quite often...

edit: example to make it clear, maybe your son logged in to YouTube on your laptop and closed the tab without logging out, so when you are searching, for google its him, because his cookie is left in your browser.