Most men who are trying to be tactful and polite say 'restroom' or 'bathroom.'

If he's more down to earth, he'd say 'take a leak' or 'I need to piss.' Piss is usually a verb over here, not so much a noun. Those are probably the two most common alternatives, although 'take a whiz' and 'take a squirt' sometimes show up.

There's lots of other more colorful ways of putting that, such as 'drain the lizard' or 'go see a man about a horse.' Both of which lead me to wonder the backstory for those euphemisms.

These apply to most adult males, so I'd say your guy could use any of them, depending on his mood and who he's around.