The National Minimum Wage for those under-18 is £3.79 per hour. (Yes, I know that, because he's not yet reached school leaving age, your son doesn't actually qualify for the NMW anyway but I'm just using it as a reference point).
So a fiver for an hour seems reasonably generous but (as long as he's happy to accept that amount irrespective of whether he's there for exactly an hour or perhaps closer to an hour and a half on a few occasions, depending upon your friend's requirements) it sounds about right to me.