What does "You tell him!" mean?

Question

  "You know how I think they choose people for the Gryffindor team?", said Malfoy a few minutes later, as Snape awarded Hufflepuff another penalty for no reason at all." It's people they feel sorry for. See, there's Potter, who's got no parents, then there's the Weasleys, who've got no money – you should be on the team, Longbottom, you've got no brains."
  Neville went bright red but turned in his seat to face Malfoy.
  "I'm worth twelve of you, Malfoy," he stammered.
  Malfoy, Crabbe and Goyle howled with laughter, but Ron, still not daring to take his eyes from the game, said, "You tell him, Neville."
  "Longbottom, if brains were gold you'd be poorer than Weasley, and that's saying something."

What does Ron mean by the phrase "You tell him, Neville"?

Answer 1

It expresses two things:

• I strongly agree with you
• I (strongly) think you are right to tell him this

In other words, it expresses both agreement and support. Agreeing with the sentiment, and supporting the speaker in expressing that sentiment.

Answer 2

You tell him means “I agree wholeheartedly with what you have just told him and support you unreservedly in expressing it frankly, to his face.”