Chances are it will be under the 1992 Health and Safety (Display Screen Equipment) Regulations (DSE Regulations) which sits under the big daddy of workplace good practice or maybe I should say minimum standards the 1974 Health and Safety at Work Act (HSW Act).
I'm not sure if there are limits or recommendations on screen sizes without looking but your brother is certainly entitled to breaks away from the screen, so he could be doing paper based tasks instead.