You'd have to note that you guys will never be able to sponsor him in the future. So if anything changes until 22 years...They will have to figure it out on their own.
He must be declared. If he doesn't want to co-operate, you can ask to exclude him. Get a notarized statement from him stating that as a legal adult, he doesn't want to immigrate and refuses to co-operate. The parent needs a notarized declaration stating that they understand that the child will be forever excluded from the Family Class and never be able to be sponsored.