The comparatively excellent grammar, sentence structure, etc. in this postcard make me think that perhaps this man worked with Jack at some point. Jack was a compositor - he set type for printing. He was listed as a "printer" when he travelled to Canada, and had worked for the County Press, Nairn, for 8 years by that time. I have found a reference to the St. Ninian Press and the Nairnshire Advertiser and County Press. It was started by John Fraser as the St. Ninian Press in July, 1892 and changed names to the Nairn County Press and Advertiser on July 24, 1897. The paper ceased production in July 1917. Reference found here.
So I don't know if the signee, John, was his employer John Fraser, or if it was another John (although the grammar and penmanship certainly don't discount him). And I do not have any idea who Dave may have been (and given their exceedingly common first names, I probably never will), but the text tells us something. It indicates that there is some culture shock. That Jack has expressed some concern about how he fits in and how people in Vancouver do things. I suspect he might be finding it hard to get a job - and I say this because the Vancouver city directory of 1911 list both of Jack's brothers and their jobs, but not Jack - although we know he was there because he came with his younger brother. Of course, he may have just been missed in the survey for the directory, because in the 1912 version, he is listed as working for G.A. Roedde (the owner of a printing company) and his brother, William, is missing. Jack and his older brother, Alexander, are both living at 434 29th Avenue E in Vancouver at this time (near Fraser Avenue, which was mentioned in the last post).
The front of this postcard is of the Scott Memorial in Edinburgh: