Here's my idea, based off a few others posted already.
In a heavily trafficked area of your fort, dig out a largish cave, with one side glass so dwarves can all see. Build a small wooden hut, with a bed and a well inside. Build a device that can flood the cave to 2/7 level of water and use it to muddy the cave, then drain it off. Build a farm plot, and dig a lake that turtles spawn in. Also trap the entire cave with spike traps- enough to hurt him, but not so much he'll die and not be able to drag himself to bed. Also make lots of masterwork meals. Dump a barrel of seeds in the house. Build a Fishery, Craftsdwarf workshop and Brewery in the cave. Finally, link up all the traps to a lever next to the glass wall, the flooding device to a lever next to that one, and create a way to dump food in, and for him to dump trash out.
Now stick Turtle in, however you want. Set him to farming, harvesting, brewing, fishing, fish dissecting, refuse hauling etc. and make him farm the farm plot, then fish. Once he catches a few turtles, have him dissect them. Then forbid the corpses so they rot. Once you have a shell, use it to make armor in the craftsdwarf workshop. If it is below a minimum quality, activate the spike traps.
Occasionally dump a masterwork meal into the cave. When he starts eating it, spike him again. While he's passed out, take the meal away (is that possible? I think you can dump it).
Also occasionally release wild animals into his cave. Maybe knock him out or lock him in the hut, build the cage, wait till he wakes up or you let him out and release the animal. Make sure it cant kill him, just hurt him. Irony- mod in turtle monsters that do gore damage.
Makes sure this is all he does. His life should be extremely boring, punctuated by the traps every now and then. Randomly flood his cave to 2/7 water, so he'll be miserable. When it drains, make him farm and fish again. If he ever makes masterpiece armor, spike him, flood him or give him a wild animal to quell any happiness he may get from it. You may also want an emergency knock-out lever, in case he goes suicidal. Don't want him to kill himself too early! If you're feeling really mean, dump his family in there too, with injuries that wont let them work (like yellow spine wounds). Then he has to feed them too. Maybe cut his farm in half so he chooses between feeding himself and his family- if he doesn't eat, he starves and they all starve, but then he has to give them less food. Maybe order him to dump any surplus food he makes, so he's always on the edge of starvation.