You get a rebate if your Catch is common. So, he might get a +1 or +2 back. Here's the other thing: how easy is it to weaponize these things? Which makes it less common and not worth the rebate.
But, you know, he might use that rebate to upgrade his power...so....
How it works is, if an attack acts as a Catch, you ignore armour and extra stress boxes related to it.
So he'd still have access to those stress boxes even in the event someone is attacking him with a Catch.
If he is in a place that might 'suppress' his Catch, causing him to lose access to his extra boxes, you might have to offer a compel to have his power be inactive for all attacks.
For instance, I had a character that had a Catch of silk. Using arrows with silk feathers, or a spear with a silk sash attached would bypass my toughness. Being in a textile factory full of silk might make it so all attacks, even regular ones, bypass my Catch. That would be in the realm of a Compel unless you are using a threshold.
My character with a Catch of Holy might lose all his toughness powers when he walks in a church just by virtue of the fact that the Threshold suppresses all his powers. Or, just being in the presence of a Church might be a compel to lose toughness and recovery powers, even if I'm invited in.