I think the death notice was probably posted by his Mum Elizabeth - James was indeed the last surviving son of HER marriage to JBG. I guess she probably preferred to ignore his second family!
Incidentally, I downloaded the certified photo of JBG to compare it with mine, and I don't think they are the same man. I'm concluding the younger man must be one of his sons by Elizabeth, either James, who died in 1877, Joseph, died 1867, Frederick died 1868 or Edwin died 1872. What a sad family!
I haven't been able to access JBG's will - do you know if he left anything to any member of his first family - Jane, for instance, who married Sydney Bird?