aagree with BB, now that he recoginzes lettuce as a food, you can use it to introduce new veggies by attaching the lettuce to the new veggie. Once he starts to munch on the lettuce, he should get a taste of the new veggie and so on. He may not like some veggies, so dont give up, try a different kind with the lettuce until he gets it and you will know which ones he favors.
He should figure it out soon enough, also, as others said , sweet potato, pumpkin, cocoanut inside , red bell pepper, zucchini..and the list goes on. good luck and keep us informed
georgie