He could use a feat to get Proficiency with a Defensive weapon. That's a point AC.
If he's a starlock, a pact blade is both a melee weapon and an implement.An easier way would be get yourself a +1 mage's weapon (AV) with the defensive property, no feat wasted.
The problem is that you must dual wield melee weapons for the defensive weapon's bonus to kick in. Implements are no melee weapons.
You can say that implements are treated as improvised melee weapons. The question is are you wielding them as improvised melee weapons or as an implement?
In both cases, our players made the fundamental mistake of paying attention to the PHB's suggested stat distribution. It's basically impossible for a class to be competitive if the primary attribute for their powers is a 50/50 split between two stats.
If he's a starlock, a pact blade is both a melee weapon and an implement.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.