"He is my son."
Translation:Lui è mio figlio.
Well a native speaker would never use the definite article here so I'd say that makes it wrong.
To expand on ms00498's answer...
The possesive adjective is used without the definite article when it modifies a noun expressing a family relationship in the singular. "Il loro" is an exception, it always requires an article. If the noun expressing a family relationship is modified by an adjective or if it takes a suffix, the article is retained.
(copied from the Italian grammar book I had at uni)