until then, the CP should do everything in their power to help the NCP with their relationship with the child. including encouraging visitations.
realistically, you can not force a teenager to go to dad's house. it will only end up with the teenager resenting dad for making her come over and mom for pushing it. it is only nature that as the child gets older that things are going to start getting in the way of visitations. there will be school sports, social events, dates, friends, etc. that will be more important to the child then visiting with dad. this is something that dad will need to come to grips with and deal with, getting involved in other ways and leaving it to the child on when they will visit dads house. the harder you both push, the further away the child is going to pull.
but as long as dad is going to push for the visits, you should help him. but try to explain along the way and help him come to terms with it.