With some caveats, it is possible to add localized fan boosters to improve duct air flow. I can recommend Fan-Tech products.

Actually getting the duct booster fan properly installed is often a problem. The ceiling below may need to be cut open, or roll back the carpet and access from above the duct. AC power has to come from somewhere.

Fan noise with a booster can be an issue. I have sometimes installed an oversized inline fan with a speed control to slow down the fan. And/or added a sound absorbing sleeve after the fan. Both add bulk to the duct.

If the room uses a five inch diameter air duct, I might choose a six inch booster fan (low noise model, perhaps with speed control), feeding a sound dampening sleeve with 6 or 8” outside diameter. The inside air path would be five inches or larger, then an adapter tapering back to continue five inch duct to room register. Or just run the six inch ducting the rest of the way from fan to room heat vent/register.

If you are lucky the duct feeding your son’s room may be exposed/accessible in the basement. The booster fan can be added down there, will boost air flow about the same.

Depending on which direction the floor joists run, it may be possible to add a second air source into that bedroom. Extending the existing air feed or tapping into ductwork from a neighboring space. All depends on how the air is moving in the room and how you want to manage/alter the air flows.

I can understand being reluctant to tear into a recently built house, and the disruption that occurs while the work is being done.