D.K.
Teething always can effect their eating. Do not give him yogurt or fruits for a while now. Try veggies like sweet potatoes or even making your own with flavor as those baby foods are pretty nasty and bland. He liked the flavor and sweetness which is so normal, however mix the cereal and the veggies together for him, so it has more texture. Him gagging is pretty normal too for that age. I used to mix sweet potatoes and any of the veggies in with the oatmeal or rice cereal and my kids liked it a lot better then just plain.
Hold off on the sweetness until he will mix it up. I used to do veggies mixed in cereal first, then as a "dessert" yogurt or fruits afterwards. Exposing him to spices and flavors younger will help him in not being so picky later on too.