Oh Cass, I am so sorry about this. I do think it's the best choice though; my mom recently had to put down one of our family cats that we had since I was an 8-year-old, and I know it is tremendously difficult; but as a true owner, we know when it is time.
All we can do, is know that we did our best with the time we shared with them while they were on Earth, and we gave them the best life possible.. I am sure he is thankful.