So his leg is possibly broken but not displaced I'm assuming. Also assuming, you might choose to splint the leg to prevent it from displacing in the future. If you choose to splint it, tip the bird upside down so he's relaxed and doesn't fight. You need to wrap the leg in something soft first, perhaps a small amount of sheeps fleece with particular attention to any lumpy bits of leg to limit pressure on the skin. Then apply a splint on each side of the leg, making each lolly pop stick two for strength, cut to length, then bandage around them. A wooden tongue depressor folded in half lengthways if you have access to one might be a bit stronger, or a willow twig split lengthways. Self clinging bandages will stay in place infinitely better than normal crepe ones and can be bought from a chemist or from your vet - they come in jolly colours.
I'm not sure whether or not I would splint it, as it would depend on the individual situation. Why did you choose to go to the vet? Was the cockerel limping? Had he been in a fight?
I think that if you do confine him to a low cage (we had to do it for a cat for ages) then he might be fine without a splint but only you can judge.