I agree with Clive, if this fellow was an American in the Canadian Army, he would be wearing some sort of insignia from his previous unit or at least a Canadian GS Cap Badge. This fellow looks like a civvie wearing a uniform, for instance, the Beret looks like crap on his head and still has the 'QM Bow' at the back. The Film and Photo title with the USA although strange by today's standard gives him some military credibility without having any rank. Look at the other fellows in the photograph and you can tell by the way they look and dress they have some military experience, this chap with the USA titles, very civilian.
The issue of uniforms to specialized persons during the Second World War was not uncommon and still goes on today. During the War wearing a uniform afforded many privilages and access that a person in civilian attire would not have and makes complete sense.
Although the Film and Photo Unit was not a news organization, the unit worked closely with the Maple Leaf staff and the CBC and I still maintain that this fellow is either a US reporter or perhaps a media specialist.
As for offocial photograph captions, mistakes are possible and only some scholarly research into the Canadian Film and Photo Unit and The Maple Leaf can answer this question.