If he is eating sunflower seeds, get a 5 gallon bucket and fill it up about halfway with water. Pour in some black sunflower seeds so they cover all the water. Lean a 2x4 up against the bucket, and put some of the wet sunflower seeds on the 2x4 . Rat finds them, runs up the 2x4, sees all the seeds in the bucket, jumps in and eventually drowns.

That kills the mess out of chipmunks too.