Hi @tommy2613 ,
Your son will need to complete a new W-4 for Massachusetts for his employer.
Below is Form W-4 for Massachusetts.
Massachusetts W-4
I suggest that in the above form, your son claims "0" on line 4 if not already "0".
If he already claimed "0" on the previous W-4 that he filled out for the employer, then he can withhold an additional amount every paycheck on line 5.
I hope this helps
