Shouldn't you be sucking mine?
I suggest skinning, its supposed to be one of the most excrutiating ways to die. Also, it would take a rediculous length of time but if we could induce a kidney stone through control of his diet that would be a decent torture to. Pulling off his finger nails is a must of course, and a salt and lemon juice mix to sterilize any cut inflicted would also be good. Second degree burns are painful, don't want to go for a third though, those stop hurting. Slitting his belly to let out his intestines but not damaging anything essential is good, I've heard that that can take up to three days to die of though I haven't looked it up myself. Just some ideas