Would the marriage be unlawful or void if the husband is Muslim by name but does not pray, claiming he will pray later to teach his children, while his grandparents were Muslims and then apostatized to another religion?

If the husband claims to be a Muslim but is negligent in his rituals, such as prayer, due to ignorance or laziness, the marriage is valid. He should be advised of the obligation of prayer and made to understand that it is not merely a statement, and that abandoning it due to laziness may lead to disbelief. If he becomes righteous and commits to prayer, there is no harm in remaining with him. However, if he insists on abandoning prayer, it is not permissible to remain with him, and a divorce (ṭalāq) or khulʿ should be sought, because his abandonment of prayer is evidence that he has not truly submitted [to Islam]. Some scholars consider abandoning prayer out of laziness to be an act of disbelief.