(Sorry, but I usually write humungously long posts, so it's a novelty to try short ones. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> )
I believe that there's some system whereby the DK's stats do level up a bit as you go along. But other than that, the equipment slots you can see are basically all you can use to beef him via wearable equipment.
You can also give him the crystal bag, and slowly add to it as you find better crstals, which increase his resistances a little. But they don't really make much difference.
There may also be defensive spells or magic that could help, but it's not something that I ever use or bother with in RPGs. Might be worth checking though.