When I take my tort outside for a walk in the yard, does he have an instinct as to what plants are unhealthy for him? Should I prevent him from eating anything I don't know?
Number one: Letting him walk loose in the yard is a good way to lose him. Eventually, some day, you will have a few seconds of inattention and he will be gone. Go ahead. Ask me how I know this...
Just build him a safe escape proof enclosure and you wont have to worry about it.
Second thing is this: Generally speaking a wild tortoise, out in the wild, is not going to walk up and start eating toxic plants. Most of the time this carries over into captivity, BUT there are exceptions. Our artificial environments can cause tortoises to do all sorts of "un-natural" things, and eating plants that they shouldn't is one of them. The safe bet is to not let your tortoise have access to any plant that you do not know for sure what it is, and that its safe. Or you can gamble and hope for the best.
well, my Radiated and Ploughshare tortoises never eat the poisonous plants even in their outdoor enclosure for example the Euphorbia. But they are wild animals so that may be why.