What does it say on the marriage certificate for Arthur's father - name and occupation?
According to his death registration he was 34 years old in 1931, so born c1897. There are 6 births registered in his name between 1896 and 1998 (and another 4 for Arthur ISAAC), but as Alan says, I can't see any on the 1901 census with father Gerald.
Doesn't help with his family, but there is this item from 1923 that may be your Arthur:
"BRISTOL ACCIDENTS ...
Yesterday morning the St John Ambulance were called to Peter Street, where they found that Arthur Isaacs, of 6, St. Michael's Hill, aged 28, had been knocked down by a car, and had received internal injuries. After first aid had been rendered, the injured man was conveyed to the Bristol Royal Infirmary and detained."
Western Daily Press, Friday, Aug. 24, 1923
Modified to add:
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