As you say you don't actually have a copy of the marriage certificate are you absolutely sure that father Robert was porter/docker on the actual certificate or have you just picked this up from looking at the various census?
The reason I ask is that I am wondering if your Joseph was actually a year or so older than he states on 1901/1911 census - that's supposing your Joseph is the Boiler man shown as married to a Sarah??
In 1911 birthplace is Liverpool, in 1901 is Garston and always living in Toxteth Park (and on both census Joseph Jones is the only Boilerman of that age in Liverpool area and married to a Sarah).
In 1891 your Joseph would expect to be found with this profession or as an apprentice and there appears to only be one candidate:
Cleopas Street, Toxteth Park
Robert Jones 48 Boiler Maker Hawarden, Cheshire
Priscilla Jones 38 Seacombe, Cheshire
Joseph Jones 19 Boiler Maker App. Garston, Lancashire
Elizabeth J. Jones 16 ditto
Robert W. Jones 13 Liverpool
Mary C. Jones 11 ditto
Herbert Jones 7 ditto
Thomas B. Jones 4 ditto
Walter Jones 1 ditto
It just seems strange to me that the only Joseph Jones bc.1874 (plus/- 2 years) connected with Boiler Maker are the one married to Sarah in 1901 and 1911 and since this was a skilled profession would have been doing this work in 1891 and again, the only one is the one above on 1891 census who is indeed the son of a Robert but this man is a Boiler Maker himself!!
Hence the question was he definitely a porter/docker or have you just picked these up from looking at the various census and trying to find your Joseph?
This Robert Jones was already a Boiler Maker when he married Priscilla Brellisford 13/7/1873 St. Nicholas, Liverpool so don't know if son Joseph was born pre the marriage as shown bc.1872 or if this is actually a mistake. Who knows?
(As an aside, there is a tree on ancestry claiming that Robert/Priscilla's son Joseph married a lady named Alice Ann Dutton 1894 but this is wrong - you can view the marriage certificate and it clearly states that this Joseph's father was a Robert Jones, who was a Baker and census shows he was actually married to an Ann and did indeed have a son Joseph bc.1872. So the tree owner has attached their Joseph to the wrong couple! Beware Ancestry trees!!)
Have just double checked marriages on Ancestry and found this marriage which is the one for a Joseph Jones 22 Boiler Maker and Sarah Stevenson 21 27/10/1895 St. Matthews Church, Liverpool - is this your Joseph?? Because, if so, it definitely shows his father to be Robert Jones, Boiler Maker which does indeed make him the son of Robert and Priscilla.
Fingers crossed this is your man.
Annette