He has to file motion to modified/terminate child support based on this current situation.

In order for your boyfriend to no longer be obligated to pay child support needs to make sure he has paid all of his back child support. If the Office of the Attorney General is monitoring his child support he can ask them for the total amount that he has remaining for his child support. If the child is still a minor, current child support is owed. If the child no longer resides with the custodial parent but the child is still entitled to child support, your boyfriend can file a motion to redirect child support to the person who is actually caring for your child.

A child on a child support order who has a child of their own isn't an emancipating event. It doesn't magically make his child a legal adult or stop child support from being paid. He will continue to pay child support until the time that the order states that support for his child will end.

The payments are supposed to be for the support and maintenance of the child. If circumstances have changed such that the payments need to be altered in some way, he needs to go back to court and file a request to modify the support order. His daughter having a baby in and of itself doesn't necessarily get him there in terms of being able to modify the payments - especially if she's still a minor. But if she's actually living separate and apart from the mother then he has good grounds to gtet a modification. (To be honest, though, unless his daughter got married or some emancipating event like that, the only thing that will change is where he sends the checks, not the amount.)