Wait, you've got him shackled in place, completely helpless, you're asking for creative ways to tickle him and the idea of opening his robe sounds wrong?I thought of the opening his robe thing, yet it sounded wrong though. Don't know why though.
I'd crave a Squilliam tickle fic.
Oh, about tickling him, putting socks on his feet. Socks make feet extremely ticklish, far moreso than bare feet -as long as you're tickling with fingers.
Also, his neck and chin/jaw are exposed. Those parts tend to be ticklish on many people, and feathers are good for that.