I feel like I want to point out here that in my experience with sensory issues, that even the best OTs know next-to-nothing about oral sensitivities.

Really, the OT should have recommended a speech therapist, as they have a "bit" more experience with sensory issues/oral defensiveness. While I believe that behavior *can* play a role, the fact is that the behavior might very well have been born out of an inability to cope with certain textures/feelings/temperatures.
I too would caution you against TO, or punishment of any kind...it will end up being counter-productive, adding stress to an already stressful situation, and teaching things that will be very hard to unlearn. Once food/eating becomes a power struggle, the child will win every time.
Have you tried looking at what he can tolerate, and trying to build upon it? He seems to be ok with crunch..can you build on that? I agree with letting him experience food without putting ANY pressure on him to eat it. Cut the banana, put a small bowl of yogurt next to it, dip one piece in, pop it in your mouth, this will show him without telling him. He won't do it, for sure...at first...but put it out a few times a week (you can finish the yogurt after). One day, ask him.."Will you stir this for me?" and then walk away....let him stir the yogurt. Baby steps. He will never put in his mouth what he cannot look at or touch....so he needs time to do that.
Some other thoughts....sweet potato fries...a few mixed with the regular ones...a few. He will get used to seeing them, and maybe eventually will try one.
I really think that of this were me, I'd try a ST...and do a little googling about food oral sensory issues. I'm going to come back in a bit. Sorry to come on strong (if I am)....I just get so passionate about this topic, and how little is known about it.