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.