Hi there.
My suggestion would be, either get him to sign a short term tenancy agreement now-- therefore agreeing to pay the rent on an ongoing basis ( think you wil just have to swallow the fact that you have lost money up to now though). This might set the scene for a future relationship which is amicable.
alternatively, if you are going down the throwing out route - please ring your local citizens advice bureau , who will outline the steps you could take.
If this is the route you do take obviously it may damage your relationship for a while, however when he devleops a more mature brain, I would think he will come round.
Whatever way you look at it, it would be best if you can get your husband to take issue with him, as he is his son, and would be better coming from him
Best of luck