You're PLACATING him?
Why on earth are YOU so scared of losing someone so cruel, so vile as to 2-foot dive your 19yo DB AND use profane language in front of YOUR DC?
Furthermore not to give a flying fuck about how badly he has been injured.
You are sending your DB off with his 'feisty' GF before she says something that makes it worse?
WTAF?
Your poor brother.
He AT LEAST deserves to see your toerag of a H come and apologise unreservedly to his face.
He deserves YOU, your DB, your parents AND his GF to tear him a new arsehole for what he has done, and what he has FAILED to do since then actually, which has utterly compounded the matter.
Your H ought to have bypassed you entirely the minute he heard what he actually did to your DB.
He should have called your DB straight away and apologised.
He ought to have come back the minute he knew the extent to which he'd assaulted him and faced him LIKE A FUCKING MAN, and say he was sorry and unreservedly so.
A takeaway and a bottle of wine? 
He's trying to win you over.... he thinks he'll try and get round you with a curry and a bottle of plonk? Make up sex on the cards too is it? just to reward his thuggishness one more way?
Your brother is worth less to you than £30?
ASTOUNDED.
If I were your DB, i'd not have ANYTHING more to do with you OR your family again, AND i'd call the Police to press charges.
He won't do that, cos he's too young to know what he ought to do in situations like these. I'm sure he'll eventually learn. 
Who the hell taught you how to treat people?