Getting rid on tenant when 6 month AST expires

I have a tenant whose 6 month AST expires on 26 December. He currently owes 1 month’s rent, which will become 2 months on 27 December. He’s not responding to my texts, etc. I completed a property inspection yesterday (he was not present) to find that he’s moved his mother into the property whilst he’s allegedly flitting between my property and his girlfriends. What’s the quickest and best way to get rid please. TIA.

Serve a s21 notice on the tenant. Not the quickest though.