Sharer owes rent, can I hold his goods ?

Rent Arrears

Hi, I have been renting a room in the flat I live in to a friend of my son. He is 3 months in arrears with the rent. We have no formal written agreement but he was paying by standing order. He has said he is going to settle up and he is due to vacate on Jan 8th anyway. I am worried he will do a runner. He has no lock on his room door.
My question is this:
If I change the lock on the front door and refuse him access to the room or to collect hbis belongings how do I stand legally ?
Would I be able to keep his possessions in lieu of a hearing in the small claims court for rent arrears ?
Many thanks ans happy new year to you all
Pete