I'd recommend Platinum or Tungsten
ETA: We had to stay away from white gold, because he would wear off the white gold and it would turn to regular gold then we'd have to go in and have it re-rodium (sp?) plated, like every 6 weeks. It was free but got to be a pain in the ass.
He now has a tungsten ring that is fine on his skin, and had a platinum one. Both with no problems.