It's bound to happen. If he's had to re mortgage the house recently he'll be paying more due to interest rates. Not mention the increase in value of the house since you've been there. I would think yourself lucky it's stayed the same for 3 years.

If I were in your situation though I would just speak to him and try and get a slight reduction. I reckon he'll come down if you've met all your payments and looked after the house as thats more important than the extra money, and not to forget the loss of letting fees and rent.