I don't know what the pet shop feed their tortoises with? It seems like store keepers only give them tortoise pet food pellets maybe not the best ones or a little treat of greens once in a while to keep cost down, if they can live with only minimums with little varieties then we must be spoiling ours rotten.If he is "seeking out" the zucchini and leaving everything else, I would leave out the zucchini for a while until he starts eating the other foods. He will, if everything else is correct in his enclosure (temperature, humidity level, water, substrate, hides etc.). Then I would "reward" him occasionally with a bit of zucchini.