I'm not going to pay my mate three months rent after being kicked out of his flat
What do you hope to achieve by refusing him the money?
It will put him into a lot of trouble because of it.
You signed an agreement with this friend, and he is your friend. You are fully expected to pay him back and withholding the rent could actually get you taken to court, so I would heavily advise that you do it or else you'll have a much bigger problem than you've bargained for.
I'm really starting to think about suing him.
I do no think that that will get you very far when you signed the lease. You should just pay the money that you owe and then move on. It will save you a lot of trouble in the long run.