Funny, I just put a 3 year old in the refrigerator from the freezer to start thawing for Thursday night supper before I opened your post.
First, when you butcher him, go ahead and cut him into serving pieces, then chill. Put in the refrigerator covered with salt water for maybe three days. I don’t do that step myself but it does help.
Then cook him. There are several ways to cook an old rooster. They all involve cooking him long and slow with added liquid. You can look up Coq au Vin (Cock in Wine). That’s a traditional French way to cook an old rooster. Just make sure you use an old traditional recipe. A lot of the recipes you get in modern cookbooks and online assume you are using one of those baby chickens you buy at the grocery. What you are looking for is a recipe that takes a few hours to cook, not something done in a couple of hours.
Chicken and dumplings is another traditional farmer’s wife way to cook an old bird.
You can use a pressure cooker. I never had but I’m sure they will work.
I have used a crock pot. Put in a bay leaf, a dozen peppercorns, a rough chopped onion, carrot, and celery, and some herbs like basil, oregano, parsley and/or thyme. Cover the bird with water and cook on low for 10 to 12 hours. By the way, if you save the back, neck, I use the wings, and gizzard and cook it this way, then strain off the liquid and take the fat out you’ll have the best chicken broth you’ve ever tasted. You can pick off the meat from this and use it for chicken casseroles, tacos, chicken salad, anyway you’d normally use cooked chicken meat. I often make sandwiches with this meat for lunch. Some people just use the entire rooster to make broth. There is no use to waste any of the carcass.
I’ll probably cook this one in a covered baking dish. Rub the thawed meat with herbs, add a little water, and cook at 250 degrees for several hours. A 5 month old cockerel usually takes about 2-1/2 to 3 hours this way. With my tough 3-year-old rooster I’ll probably go 4 hours before I start checking him.
He's around 18 - 22 months old, and I imagine he will be too tough to eat. Does anyone have any tips on how to tenderize the meat after slaughter or will he just do as soup or dog food? (Hope this is the right section to post this!)
We slaughtered him yesterday and he has been in the fridge brining since then. I'm erring on the side of caution because the owner of the farm recently raised some meat chicks free-range style, and they took forever to get to size. He finally slaughtered and sent us 4, and straight into the pressure cooker they went, 40 minutes, another 40 minutes, another 40 minutes... That meat was TOUGH, not firm... So I was a bit put off by that. 
