Answer:
The Liquid Crystal Display lighting of the monitor is most likely faulty.
Step-by-step explanation:
By connecting his computer to an external monitor and observing that the brightness of the external monitor was fine, we can deduce that the dimness of his computer was not a software issue but that of hardware. That is the computer monitor itself.
The most probable reason for the dimness of his computer monitor would e an hardware issue occasioned by a faulty LCD - Liquid Crystal Display or LED, the back light powering the LCD of his computer computer monitor would have to be replaced by a qualified technician.