"Εσύ δίνεις υπερβολική αξία σε αυτά που λέει."
Μετάφραση:You give too much importance to what he says.
Yes, you can use either. The plural is more common though, especially when refering to what he says in general. The singular fits only when refering to what he says/said about a particular issue.
I am completely dependent on help from Duo friends to lead me from the fog into the sunshine, thanks