My husband actually loves having his balls stimulated and (as sometimes I concentrate on the main event forgetting about them – maybe because while I do enjoy quite a lot giving him oral, playing with the balls is a chore for me I only do out of love ;)) he even asks for me to involve them more. One of the techniques I use is a sort of Baoding balls (aka “Chinese balls”) exercise: I obviously don’t rotate them that much, but I do imitate that movement therefor applying a rhythmical rotatory massage on them. Pulling them is also ok with him.
He enjoys having both his balls in my mouth, either sucking or just teasing with my tongue.
We did try vibrations out, even from my wand, and he does appreciate the sensations lately. I confirm what was already mentioned above: he also prefers a steady vibe (you just experiment with the intensity of his liking) and not the patterns.
GENTLENESS with them is a must, I can’t stress this one enough, whatever you choose to try out!