Wifi supports a bunch of different speeds *from the same set of radios*. If the device has a problem connecting at higher speeds it will try lower speeds. There are a lot of things that impact the quality of a wifi signal; distance from the radio as well as objects between the radios to name just two. Higher frequencies such as 5GHz will be impacted more drastically by walls/floors than lower frequencies such as 2.4GHz for example.
The first thing to do is determine what the signal quality is in his room. There are a few apps for a phone that will give you wifi signal strength. Antenna orientation can also impact signal strength so if you find that the signal is weak, try moving the wifi device to see if the signal strength improves. If you determine signal quality is good then more information will be needed resolve the problem.