"You thank him."
Translation:Tu le remercies.
The verb "remercier" takes a direct object, not an indirect object. You would then use the direct object pronouns (le, la, les), and not the indirect pronouns (lui, leur).
In other words, you would say "tu remercies" + person, and not "tu remercies à" + person. Replacing "person" with a pronoun, it's "tu le remercies" and not "tu lui remercies".