Download a WiFi checker app onto your phone and see what the signal strength is like in his room.
Assuming it’s crap then you have a few options in order of good to bad.
1. Get a mesh system. We have 1 router upstairs and another downstairs. Gives us good coverage in our house.
2. Power line over ethernet. You can get ones that have a pass through power socket so you don’t lose the socket. We also had a set of these in the old house. Worked well across the upstairs and downstairs ring. I assume your son’s device can be hard wired.
3. WiFi repeater. Tried various incarnations of these and found them to be a bit crap.
If everyone upstairs is having WiFi issues go for option 1. If just your son then go for option 2.
As a real quick and dirty use a mobile phone as a WiFi repeater. Connect it to the home WiFi, setup WiFi tethering and leave it at the top of the stairs. Then connect laptop to the phone tethering network.