I have read through the FIFA laws of the game and can see that the half time interval says:
Half-time interval Players are entitled to an interval at half-time. The half-time interval must not exceed 15 minutes. Competition rules must state the duration of the half-time interval. The duration of the half-time interval may be altered only with the consent of the referee.
We play amateur U18 football in the UK, with 45 minutes per half. I have looked through our leagues handbook and can find no mention of the duration.
Is it reasonable for the referee to insist on only 5 minutes half time interval?
The above rule was taken from the FA laws of the game.