This 'child' is old enough in UK law to marry and have his own children, therefore surely old enough to take responsibility for his actions, he is a young man not a child, he would not even be classed as an unaccompanied minor on the major airlines.
DS1 and 2 regularily travelled with DD 7 years younger than them, they were classed as responsible adults.
If the young man had committed a crime he would be old enough to be tried in an adult court, somewhere a line has to be drawn, the government decides how you bring up your children or you do.
Two DDs of friends have just returned from a trip to Mongolia to vist family friends, they are 15 and 17 and travelled quite happily by themselves.
When DD visited friends in Morroco at 15 Morrocan airlines classed her as an adult.