Once more, with feeling.
Putting this horse down with a spark in his eye as a happy young guy is going to hurt like hell. The only thing worse? Waiting until he is in pain with no prospect of relief. Horses only live in the “right now,” not the future. This guy isn’t thinking of all the things he’ll miss, he is thinking of how he feels in his body right now. And if he does go down the road of constant pain, he won’t be thinking back on all the times he felt good and being grateful for it - he’ll just be thinking of how bad he feels right now.
It is a huge, huge gift to him to make sure that all of his “nows” are comfortable and pain free. He will not know the difference. You will, and it will suck REALLY bad. But it will be the right thing.