Why can't you use lad? It sounds better than guy and shows that he is bot just a boy.
"Kille" is probably more used than "pojke", and "pojke" can only be used for children. A twenty year old male person would definitely not be a "pojke", not even in newspapers or similar texts. The newspaper would probably write "ung man" (young man) about a twenty year old man.
You could use "kille" for children as well and that would probably be considered a bit more "cool".
Yes, much like its female equivalent "tjej" can be used to mean girlfriend.