Left behind because of Mango flight overbooked

I had booked a Mango flight for my son in February for the 30th of March 2018. On Friday the 30th, my son was supposed to leave Cape Town at 11:40 on flight JE 348 from Cape Town to Durban, and arrive in Durban at 13:40. When he got to the airport, he was told that the flight have been overbooked and was put on the 15:00 flight and only got home after 18:00hrs. I had initially paid a higher than normal fare for the flight(high peak fare) so that my son could attend a Mass service(AS CATHOLICS) , which takes place at 3Pm worldwide. which my son could not attend, he was also going to participate in Alter service during the Catholic Mass and could not do that, which is something that a devout catholic cannot miss. The only thing my son got was a R60 meal voucher, which was not compensation enough for the inconvenience. My son was not given an option to say if he was in a hurry or will stay behind on the overbooked flight. It is the airlines fault that the flight was overbooked and we had booked the flight almost two months in advance. I think it’s only fair that Mango compensate my son with a free return trip to Durban for future travel. I have written countless emails to Mango and they have not responded and not even acknowledged my emails,