The easiest way is to call the airline directly and tell them you want to buy a child's ticket. Hopefully they will ask if the child id travelling alone and if so what type of arrangements are you making ( ie unaccompanied minor). you can also call a travel agent they can provide the information needed to the airline.
You should be able to get a quick answer for this from customer service at any airline site. Otherwise, call them up and ask about it.
what you do is you call the airlines booking # and then you can get an unnacompanied minor ticket so that the airline knws that he is flying alone so they can tke good care of him
Then visit your Airlines' station available in your region. Your 11 year old can be a UM in Cathay Pacific and every airline has this choice and also the child can choose the food he/she wants to eat at the airplane