What does it mean to "squeal on someone"

What does it mean?

He can't squeal on him.

From American Heritage Dictionary via Wordnik:

squeal Slang To turn informer; betray an accomplice or secret.

Here's some synonyms from The New Oxford American:

inform on, tell tales on; report, give away, be disloyal to, sell out, stab in the back; informal rat on, rat out, snitch on, put the finger on, finger, sell down the river.

You can find some good examples of it in use at Wordnik.

It started off as criminal argot, but has worked its way into the general vocabulary to mean anyone who "tells" on someone for doing something "wrong". A sister can squeal on her broother, a student can squeal on another student, and so on.