"Your son believes this farmer."
Translation:Wasz syn wierzy temu rolnikowi.
Vast majority of masculine nouns have -owi.
-u is used by neuter nouns and few common masculine nouns.
rolniku? It's either locative (Mówię o rolniku - I'm talking about the farmer) or vocative (Hej, rolniku, co tu robisz? - Hey, farmer, what are you doing here?) form.
Most have masculine nouns have the ending -owi in their dative form.