The feels on this are real, I dealt with this similar issue years back. Lots of love and advice for you already, but 1 thing is missing you need to hear:
This is not your fault. There are very few signs that would be seen by us that would indicate anything is wrong. Cats (and other pets) have instincts to hide the pain from predators until they are healed. No matter how much you love them, pain is weakness that is exploited in the animal world, even by trusted animals.
I hope he heals up, and I’ll be giving my cat a little extra love today.