Try Earthborn Coastal Catch or Meadow Feast. They are a decent price for a great food. Also pick up some digestive enzymes and probiotics at the health food store and canned pure pumpkin. Give him a heaping teaspoon of the canned pumpkin with all his meals while you are transitioning and for a little while after. Give the enzymes in every meal too, enough that he is getting an adult dose every day for at least 2 weeks after you have finished transitioning. Give an adult dose of the probiotic every day. You can do this any time you transition foods as long as you need to.
You described tapeworms exactly, your vet is not guessing at all. It only takes eating one flea to get them, so they didn’t have to have noticable fleas. And he had to have eaten that flea fairly early on for him to be shedding segments already.
