The mother can get a protection from abuse order against her son is he is abusive or threatening.
Mom should look for a local domestic abuse center they can tell her how to get one and help her.
Or go to her local courthouse and ask about a "PFA". On weekends or when the Court is closed the order can be issued by the night District Judge. The police will know how to get one.
Mom do not hesitate to call the police if need be. You need to stop this now. It will only get worse for your son as he gets older.
Contact the abuse network in your area. There should be a hotline in the yellow pages or call your county courthouse for the number. They can help you decide if a protective order (PFA) is appropriate. One other thought: where is the boys father? If he is not in the house, is he an option to take custody? Best of luck.
Note: my comments do not constitute legal advice nor are they intended to establish an attorney/client relationship.
315 N Market St
Port Royal, Pa 17082