Sounds sensible, on balance his arrears split between the other three of you isn't exactly going to ruin you.
Play the long game, and file a claim against him in due course... it will seriously hamper him if he can't get a reference for other properties because it comes back with a CCJ for a rental agreement he decided no to pay (you may want to spell this out to him).
Just make sure you have a forwarding address, or ideally a parental address to file against, and if he doesn't file a defence statement within a set time the claim is filed and and/or is a no show for the preliminary you can request the judge to rule against him on that basis alone.
But do formally give him 30 days notice to pay, clearly stating your case -remember you have to be seen to be very reasonable here if it ends up in court.
As for people suggesting to just kick him out, that's unlawful, there's no two ways about that. As I said in a previous post, all he is guilty of at the moment is being behind in his rent, the alleged refusal to pay any additional rent is nothing more than rhetorical at this point. If you do this it would go one of two ways -
If he's not organised you may never see him again and you'll end up with whatever junk he left behind.
If he is organised, of he has a relative who takes the fight up on his behalf you could end up in serious bother.