Consider adding a through wall vent to allow return air flow out of your son’s room into the hallway.

One method I have used is to cut two vent openings in the dividing wall between hall and bedroom. One vent is low on the wall, the other is high on the opposite side of the hollow wall. The air can now flow through the room side vent, through the hollow wall cavity, and out into the hallway.

If sound management is a concern and the offset vents allow too much sound transmission, I add a layer of sound absorbing material (fiberglass ductboard or fiberglass duct liner) to the inside surface of the wall cavity. My source for this is sold for trade/commercial duct sound absorption. It is a dense matteted fiberglass material, designed to be installed inside metal air ducts, such as you might find on a large commercial building.

This product is commonly sold for filling hollow wall cavities and dampening sound. the problem is you need to leave a large channel open within the hollow wall for air flow. Fill it too full with sound material and air flow will greatly decrease.

Another source is sound absorbing material sold for automobiles. Some have rubber backing and foam layer, with self adhesive backing. Worst case is you cut open the wall cavity to install the sound treatment, then put drywall up, tape and repaint.

In one place I installed a vent allowing room air flow into a closet, then another vent allowing the air flow out the back of the closet into the hallway. Closets tend to be good at absorbing sound, especially if full of clothes.

A method that is common in home building, which I dislike, is undercutting room doors to allow more gap for air flow. Not only does this force the exiting air to be from the lowest part of the room (coolest air in son’s room), it looks terrible and allows a lot of sound to pass under the door.