If you can spare a few days to deal with this problem, clear away all other potential food sources in the area and then put down plenty of bog standard rat poison. Give them loads of it. They gobble the stuff up and then die about five days later.

You can augment with a rat trap or two with a little peanut butter on the trigger, but these things seldom tempt them, and they can be nasty if they spring on your fingers.

Some shops offer a kind of sticky paper for catching them, but that stuff is unbelievably cruel, and not at all reliable. Best thing is to just dose him up with rat poison.