Is boy chlopiec or chlopak
It's not exactly the diminutive form, it would be: chłopiec -> chłopczyk and chłopak -> chłopaczek. But in general it's used as you've described.
And yes, chłopak is also usually used as boyfriend (and dziewczyna as girlfriend), not chłopiec and dziewczynka (which is the diminutive of dziewczyna). I can imagine my grandmother saying mój chłopiec jest silny (my boyfriend is strong) when she was young (even about 20) - but nowadays a girl would say mój chłopak jest silny.