Hi
I don't know anything about the rules for acquiring a Mariners Certificate, but what you say does make some sense - i.e. overstatement of age.
If his real first name was William ....
Like before, I can see nine possible candidates in the 1841 Census. Seven can be discarded because the father's name doesn't appear to be John. Number eight is with mother, no father in sight. The ninth looks possible and a little too old:
1841 Census of Wool, Dorset
HO107/279/14 folio 11 page 16
(Address not specified)
John James, 60, carpenter
Fanny, 52
William, 18
James, 16
Fanny, 15
Mary Ann, 13
I think you may have to take the long way round. Have you identified any brothers or sisters who married before 1881? If so, their marriage certificate might reveal their father's occupation. You'd have more to go on then.
Cheers
Dave