I'm sorry for it. I made a mistake. The correct answer is (b) to have discovered. Couldn't you teach me why (a) to discover can't be the other right answer?
I see why it can't be (a) to discover. Then, I think that "him" indicates Mr.Tollin. Why did not the questioner write "Mr. Tollin, the account manager who was hired recently, appears to have discovered the perfect profession for himself"? Thank you in advance for your teaching.