Do you know what kind of paint they used? After they finish painting, I'd go home and assess the situation. If you can't smell any lingering odors, I would try and open all the windows if you can (I know it's winter but maybe if it's sunny for a few hours) and then after two or so days it would probably be fine to bring him back. If there's significant lingering odor, however, I personally would wait 5-6 days if it's OK with your son. I'm paranoid about these kind of things so 4 days would probably be fine but..