Hi Evelyn

Have you found Hugh in a later census. The 1891 shows him as age 48 on the index (hard to be 100% as this is where the enumerator has decided to put his counting line!) Although I would say, looking at the image, it does look like a 4 rather than a 2 comparing other 2s on the page.
There are only two births showing in Greenock between 1860 and 1866
HUGH MCDONALD Birth: 05 JAN 1863 Middle Or New Parish, Greenock, Renfrew, Scotland
HUGH MCDONALD
Birth: 31 MAY 1864 Middle Or New Parish, Greenock, Renfrew, Scotland
Both births have parents Donald McDonald and Mary McDonald.
Maybe worthwhile looking at birth years of circa 1843?
Regards.
Monica