He presumably started his training to be a pilot as Stormy Down was an armament training school for the Royal Air Force. It was the main base for the area and directly connected to RAF St Athan using a long taxiway stretching around 30 miles from each other.
Although I haven't obtained Bobby's service record, the likelihood is that he fell ill during his training and was unable to qualify as a flying office like his brother. He was diagnosed with leukemia and died in the RAF hospital, St Athan, on 18th October 1944 and, like his father and brother, is remembered on the CWGC memorial at Wendover.
He was buried with his brother. The gravestone (photo below) says :
"Great Pals - To their gay and gallant spirit a happiness which once has been is imperishable. R.I.P."
George Musgrove must have been devastated at the loss of his two young sons. He himself died less than eight months later.