It appears you have full custody already in that he has not seen the children in 18 months. Child abandonment is a criminal charge for completely abandoning a child, such as leaving a child on a doorstep. Choosing not to exercise custody rights is not defined as abandonment. If you file a custody action, he may seek to exercise rights that he has not been exercising lately simply because you provided him with an opportunity to do so. Unless it is your desire that he become more involved, I would not take any action unless he suddenly tried to exercise custody as though no time has passed since he last saw the children in which case you can file custody to require counseling to be involved before he has regular contact.