chunky food

Eric is gagging on textured, chunky foods. He is fine on the pureed foods( in fact, he loves those) but any of the size 3 foods (especially the Gerber brand) he gags and vomits. I've also tried some spaghetti noodles (table food) and he just plays with it.
I've been trying these textured foods for the past 4 months or so. Sometimes Eric starts to eat a bite and it goes down and then he acts like he wants more and so I give him a little bit more and just when I think that it's getting better and he's getting the hang of it, he throws it up.
I've told the pediatrician about this and he says that Eric isn't ready if he is gagging. I'd like to try feeding Eric in his office to show him what happens and then see what the doctor says. I plan to talk to him again at Eric's 15 month checkup next month.

I'm not sure why I feel so frustrated. I guess I feel like I'm the only one who has a toddler who has trouble eating.
Has anybody had this trouble with their kids? Or does anybody have some ideas?