Don't have a grey horse, just wanted to comment on your boy. He is beautiful! That has to be my favorite color of grey. I think he would look great in any color of tack.
If your going for classy: IMO black looks FABULOUS on steel grays(:
If your going for colerful but modest: hunter green(looks good on everyone, but still fun)
If your going for FUN AND CRAZY: go for purple. He looks macho enough to pull it off(: