My sincerest sympathies. You can comfort yourself with the fact that you're doing the right thing for him by ending his illness and whatever discomfort/pain there may be.

We had to do the same with our little guy last year. He had kidney problems, and we gave him a saline drip twice a day for about a year. He never complained. But at a certain point you have to consider that you're just prolonging their suffering, as hard as that is to accept.