Yes, it is. When we exercise we are burning more oxygen and with sickle cell trait, sometimes those sickled cells that co-exist with normal red blood cells don't carry enough oxygen so it can exacerbate pain symptoms. My recommendation would be to have your son try light to moderate exercise routines in 40-mins sessions first and increase the intensity of his workout every two weeks. That way, you're communicating to your body and cells to increase blood flow in small stages.