ancient latin?

Question

if you want to say
cornelius has one son.

would you say "unus" or "unum"

Answer 1

unum filium.

The 'unus' is declined just like any other adjective.

In classic Latin, it would be more like

Unus filius Cornelio est.

The idea of 'having a son' as an object came later.

Answer 2

Depends on how you say it. "Habere" is a transitive verb, so :
"Cornelius unum filium habet"
The classical version is "Cornelio filius unus est.", where "filius unus" is the subject of the sentence.