traveling

I am from Malta, i lost my mum 2years ago because of breast cancer.About 3years ago my dad was diagnose that he had well difference lypo sarcoma ,he `ve done the operation and radiotherapy.Hegoes to his doctor every 6 months for regular check ups.Now,his doing well,the problem is ,he wants to go for holiday in north uk to visit his relatives but he is afraid that because of pressure and different atmospher when you are on the plane may the cancer give him the chance to come out again. it takes him 3 hours to go and another 3 hours back. I wish someone can help. thanks and god bless.,