caught my partner cheating at work

I thought that my partner was cheating on me. So I started following him to work and anywhere he went. Then one day,  I drove up behind his work truck and caught him in the act with some slag bonking in the back of his truck. He then says to me when he gets home that I was just imaging what I saw and that he would never cheat on me. Even though i caught him fucking some chick. Thats crazy I said I saw you doing it with someone else. And to this day he still denies it even though I caught him.