Wyll's stats do not support blade pact warlock. He is okay as chain/tome, but that doesn't really fit his back story.
To make him "work" as written they need to have his patron give him +cha to attack and damage with the rapier in addition to the rest of the fiend kit. That would fit the story nicely, as his "deal with the devil" would give him real fighting prowess well beyond his actual abilities.
Player could then still patch his awful AC with the moderately armored feat at level 4.
