A couple of possibilities based on your limited details. 1: There is something noted in his passport or a problem with his passport, 2: The UK has placed his name on a watch list, that is shared with other governments.
He should check with the nearest passport office to see if his passport has been canceled or blocked in any way or if there is a physical problem, like non readable swipe code or bad chip (if it has one).
If there is not an issue with the passport itself, then he might need to check with the government to seeif his name (or someone else with the same name) is on any watch lisr or wanted list. Not familiar enough with the UK goverment to suggest a spe ific department.