Hi Val,
It was Charles
Herbert JOHNSON who was born in Sydney in 1884, son of Charles H or N JOHNSON and Harriet Ellen MARTIN. He committed suicide in 1919 and from death notices appears to have been unmarried. Sisters May, Ellen and Gladys - married surnames REID, BOURK, (O')FLAHERTY. Harriet died in 1924, and Charles in 1938.
The Howard middle name is only recorded in the 1943 marriage and the 1957 death, which appear to be YOUR Charles.
As for Charles Howard JOHNSON'S son, it will be quite difficult I would imagine with such a common name. I wonder if he was also given the middle name of Howard? I had a look for
*** on the nominal roll for Vietnam as he would have been of an age to serve, but none born NSW at the correct time. There are lots of internet sites for finding missing people, so maybe post on a few of those. (Everyone googles themselves at some stage!) If the death cert indicates that his wife was still alive then maybe you can have the few notices from the Sydney Morning Herald checked as well (the link was given earlier), keeping in mind that she may have remarried. Letters to local newspapers can also be very successful. The marriage cert will give details of the bride, and maybe it would be easier to chase up living members of her family, as the name is a little more unusual.
Good luck with your search

Debra