Did you import your 1099-INT forms from an online account (bank or brokerage)? Is your son's account visible to you when you log on to your account? If the answer is yes to both of those, then his interest income was likely imported at the same time you imported yours.
To fix, try this: go back and delete the entry for interest income from that bank or brokerage firm. Then begin the process to import the 1099(s) again. This time watch carefully for a screen with multiple accounts listed. There is a check box to the left of each account. Uncheck the box next to your son's account, then continue importing your account(s).