Legal Question in Family Law in Massachusetts

You are saying your ex has a girl friend who he has never met? That is strange in and of itself.

You can seek a temporary modification and an inspection by child services to make sure there is an appropriate environment for your son stay overnight once they have moved in and settled in the apartment.

I am assuming your ex's apartment is a two bedroom home and as such the two preteen girls will stay in one and your ex and the girl friend the other. The issue is where your son will stay overnight. The court may be interested as well, given the number of people in the home and none of whom anyone has met apparently. Make it clear you are not accusing anyone of anything, since you have not met the girl friend or her daughters and your son has not met them either, so you cannot make an accusation. This can be a significant emotional adjustment for your son and as such you want things to be done slowly for the benefit of your son and his father.

You might ask your ex not to agree not to keep him overnight for the first two or three months after girl friend and her kids move in so your son can adjust to this new environment. Have your son continue to visit with his father.

No one can predict what Probate Judge will do, but as long as the request seems reasonable you may get what you want.