The Y-chromosome springs to mind...or does that make him only male?
I'll go with the definition researcher David Gilmore gleaned by looking at cultures around the world: a (manly) man is brave, economically successful, responsible, generous, sexually capable, procreative, and sociable with other men.
I would add to that the parameters of dignified, protective, resilient, charitable, traditional, and owning a full and manly beard.
I say a man is an adult male who takes repsonsibility for his life. According to your definition, Will, I'm not a man.
It wouldn't be politic to ask why not!
because I can't procreate, am not economically successful, or sociable with other men.
We all have our limits. I'm not very sociable. I'm not rich, but surely we should all aspire to have enough to support our lives' missions, whatever that is. Take what fits, and set priorities.
I'll agree with the basic answer of the y-chromosome. After that there are only qualified states: manly man, sportsman, the gentleman, a good man, ladies man, and so on each with their +/-connotations and stereotypes.
What qualities make someone a man?
A formidable stash of the best porn.