Trip him. A lot of armor benfits go away when you're sprawled...
Can you give an example please? I'm struggling to find any benefit that is lost.
Not -too- much is lost when sprawled, but DR from Armor is halved:
Damage Reduction granted by the armor. This decreases to 1/2 (rounded up) when the wearer is held, helpless, pinned, sprawled, or unconscious.
Fortunes of War wouldn't be affected, but the Armor feats and fittings very likely would, dropping his DR by 4- DR 7 is still pretty decent, however.
You could, of course, always just throw Acid at him, all those damage saves the armor gets aren't worth a whole lot in that case.