Good for you. It's a tough decision to make. There is nothing wrong with butchering an extra rooster. (When I had several hens go broody this spring, I was hoping for more cockerels than pullets for that very reason. Freezer fillers! I know you can eat pullets, too, but I'd rather not.) As a matter of fact, I see this as good, responsible flock management. It's hard to find a home for extra cockerels/roosters. At least one that is willing to be a "permanent home". And around here you can hardly give them away, let alone sell them. I figure, why would I put the time, effort and money into raising a bird just to give it away to someone else to eat? I'd rather eat it myself.
Do you have a pressure cooker? That's one way to tenderize an older chicken. If he's less than 6 months old, I don't think you'd even have to do that. A crock pot is a handy tool, too. The carcass can be used for broth after all the meat is picked off it. That can be frozen or pressure canned. So many good uses for chicken and broth!
Let us know how it goes. We've been butchering chickens for years, and it's always kind of a sad thing for me until we get started. Then it's just something I know has to be done, and I deal with it. I'll admit - I have never actually killed one of my chickens myself. We use the hatchet and stump method to decapitate them, and I'm so uncoordinated, I figure I'd amputate my leg if I tried it. If I were doing it myself, I think I'd use the cone method, even though I don't think it kills as quickly and humanely as decapitation. Wishing you all the best. And again, kudos to you for making a hard decision.