What do you mean by "add his conviction to every attack and defense"?
You don't mean he's literally adding the skill value, do you? Because that's not how it works. Complementing a skill means you get a +1 to the roll.
And the power as written does not mean "use it on any roll that fits your high concept." It more means, "Describe a specific purpose in this situation that aligns with your high concept; invoke your high concept, and you get a bonus for acting toward that purpose."
So he'd need to first decide on a specific purpose (and as GM, it's well within your purview to say that general defense of your allies is way too broad), and then spend a fate point. Further, if he completes this purpose, takes a compel drawing him away from the purpose, or buys out of a compel meant to keep him on purpose, the effect ends. It's by no means a free lunch.