The marriage certificate would prove very useful, I think?
Because it will have his age, and his father's name.
But a possibility (to bear in mind!):
Births, June qtr 1900
Dudley Registration District vol 6c, page 26
Rock, Joseph Seymour
The hyphenated surname might be explained by an illegitimate birth?
There are a plethora of Joseph Johnson births - you really need his father's name to trace any further.