I would tell him you are willing to pay an increase in line with inflation

if he says no , tell him your gonna stop paying the rent all together and claim squotters rights and live there rent free for about 2 years ( that is roughly how long it will take him to get you out )

This will work as a good friend of mine owns some flats and this is exactly what happened to him and he tells me that the tennant has more rights than he does when it comes to squotting and it was the biggest nightmare of his life and there was nothing he could do other than go through the courts and it cost him a fortune in fees and lost rent , he tells me know that having a good tennant who pays on time is the most important thing rather than an extra £20 a week

He will shit himself