What about 'guy'?

@DM_with_secrets My instinct is that "guy" would work in a first-person narrative, but would be a tad too informal for third-person.

"Man" could work if there was a romantic interest?

Would it make sense in the narrative to refer to him by his name or his profession?

Am I mistaken in believing that "boy" is commonly used for any male who is young and unmarried, regardless of whether he is actually an adult?

I propose "dude".

Surely he's a lad.

What about changing the way you refer to the protagonist depending on how your female character feels about him ? He could be a boy when he acts childishly, or a man when he shows maturity ?

@DarkMalthorp - I'd say yes, you are mistaken. I believe a 25 year old unmarried man would get seriously offended if you called him a boy. It can only be read as putting down.

@Davor - Oh dear, I may have inadvertently offended a number of people :/ Does the same apply to "girl" and "woman"?

@DarkMalthorp - much less because women idealise youth while men idealise masculinity/maturity. Women often call each other "girl" even if they are 50 and have 4 children, but for men (especially young men insecure of themselves) the implication that they are a child to you would be much more offensive. Of course, men will still say things like "that's my boy", but context is king. If you stop a 60 year old to ask a question and he says "yes, boy, how can I help you?", well, that's because he's 60 and he probably calls everyone under 40 a boy :D

The title of the question asks about how you, the author, should refer to him, while the body asks about how your character should refer to him. Those are 2 largely distinct questions. It is perfectly acceptable (and not uncommon) for an author to not speak (write) in the same way as their characters do, if this makes sense in the context of their story.

@DarkMalthorp You can use "boy" to refer to any young adult male if you're specifically trying to offend them, if you're old enough (but even that's probably just a tolerable level of offensive) or if you're trying to refer to them in a shy or cutesy manner in a romantic context ("I met this cute boy today"). "Boys", on the other hand, is more commonly used for adult males in a more neutral way ("one of the boys", "boys' night out", etc.). On a related note, "bachelor" refers to any unmarried (typically not too old) adult male.

Is this identification of the male character part of internal/narrative portion of the text or is it dialog? Does she think he's a boy/man? Does she identify him as a boy/man to cops? Does she identify him to his face?

@Davor Please don't call a woman "girl" when you wouldn't use "boy" for a man in the same context. Some of us actually will be really offended. google.com/search?q=calling+women+girls

@DarkMalthorp Using "girl" for "woman" is just as bad, please see previous comment. There are contexts where it's OK, but if you're unsure then it's much safer not to.

@user3067860 - yes, well, tell that to women, they are the ones calling each other girls.