One of the Fighters in my party has an AC of 21 at level 5. He wears chainmail which gives an AC of 16, has the Defense Fighting Style which gives a +1 to AC when wearing armor, and a +2 shield (found in an ancient cave) which adds an additional +4 to his AC. As the DM, I am having an issue with any number of creature hitting him. I understand that I allowed this to happen, but I'm starting to regret it. How can I at least give this Fighter an actual fight without being unfair?