Change of return flight from United Kingdom

I booked a return flight for my son who left for the UK on the 11th of January 2015. I booked the return back for 6 months later, knowing he might return later. He started a job within two weeks of arrival, which was from 8am until 5 or 6pm in the evening. As the time came closer him and I tried to change his booking online for January 2016. It would not allow us saying we had to contact British Airways direct. Which I did got hold of them here and asked them to please change it explaining that my son had been trying to get hold of them too. I must mention that I had been trying for a week to get hold of anybody who could help me or simply anybody just answering the phone!They then told me my son had to change the booking and when he tried again it said that he sector had been flown. I have emails that was written to British Airways Customer Service whereby they said that they would deal with this but nobody ever came back with a response. My son has now tried again and they told him that he had flown on the 11th of August 2015 which was impossible as he is still in London and they can check to see. They told him that they would refund him 30 pounds if he books with them again.