I'm sorry to hear about Cookie
He is your rooster and you have evaluated him to determine that he is suffering and his time has come.
I respect your decision.
If a vet is an option, they can put him down for you. If not, then let your husband take care it since he has offered.
I know of nothing that you can give him, sometimes there is no easy out and you just have to get to the task at hand. I don't think any of us enjoy putting a much loved chicken down, but it's our responsibility to do what is right by them no matter how much it hurts the heart.
If you have photos you would like to share of the damage to his legs, others may be able to make suggestions for treating him. If the damage is severe like you mention, you would likely need to cage him in a place that is warm and tend to him several times a day. Sometimes that is not an option either. It takes a lot of time to rehabilitate an injured animal IF he can even be rehabilitated at all. Family, work and personal commitments all need to be considered before you take on
long term full time care of a sick animal.