Your husband can join your Family plan with his existing account. That way he can keep his library.
His billing will switch automatically - he doesn’t need to cancel anything. When he joins a Family plan with his existing Spotify account, all subsequent payments on his account will be made by the plan’s manager (the person who started the plan, in this case that would be you).
Note: If his current Premium plan is associated with a partner, he needs to cancel with them.