Is there a single word for a person with great interest in photography but not really a photographer? who is an enthusiast for photography or atleast who is going to be one soon?
I'd suggest "shutterbug."
The Free Dictionary defines this informal word as: "an enthusiastic amateur photographer."
In a more generic sense, calling them an enthusiast works nicely. It includes those who are "going to be one soon" (as described in your question), and if you want to step it up to include a much more active participant, you can qualify them as an avid enthusiast.
Semi-pro would refer more to someone who works as a photographer part-time but that isn't really his main occupation, while pro-am would imply a sort of professional-level amateur, an enthusiast who's as good as professionals in his field.
There´s the term ¨protog¨ for professional photographers, I´d say ¨amphotog¨ works pretty well for amateur photographer.