Formal:
* Masculine
* Manly
* Virile
* He (pronoun)
Informal:
* Guy
* Dude
* Lad
* Fellow
Other options:
* Gentleman (formal, polite)
* Boy (young male)
* Man (adult male)
* Sire (formal, archaic)
The best synonym to use will depend on the specific situation and your desired tone.