On BA, rows at the rear of the aircraft are held specifically for UM's so that this issue is less likely to arise, also the crew can better keep an eye on them.
With BA, unaccompanied minors are those for whom the appropriate fee has been paid for BA staff to look after them. This applies to all children under 12. BA won't carry any child under that age without them being classed as a UM and taken care of from the time they're left at the airport to the time they're collect by a parent/guardian at the other end.
It seems this was not an unaccompanied child or they would have been sat in a more appropriate seat in the first place. So was either over the age of 12 which is not classed as an unaccompanied minor, or was travelling with parents who could have moved to prevent this problem.
If he was with his parents, perhaps the cabin crew should have asked the parents and child to rearrange their seating, rather than what happened.
If the former the child should have been seated with the other UM's at seat allocation/check-in. The age of the passenger should have been known.