Hi there, I work with a boy, whose eating habits sound almost identical to those of your son. However the good news is that we’ve just had a huge breakthrough with him and since then he’s trying new foods thick and fast (well comparitevely). No magical solutions I’m afraid, the process took about 2 years (we had tried other things before this with no success).
Our tactic was one of imitation and huge reinforcement, in the beginning all we would ask is for him to copy us picking up the piece of food, or breaking it up for example, we made no suggestion of putting it anywhere near his mouth and for copying us we rewarded him hugely.
Once he was comfortable with this we gradually worked up to him touching his lips with the food, touching it with his tongue, putting it in his mouth, biting, licking etc. all done very slowly and never being a negative experience. We eventually got to the point that he would chew on a piece of food. We stayed at this point for a long time, eventually we got him to chew on a piece of apple for long enough that it disappeared. We went crazy!! And from there things have gone great, although still slowly for quite some time.
He now eats cold meat, chicken and fish, 2 vegetables, fruit, nuts.
The key thing was that we never pushed and that we had good reinforcement. The focus for us shifted as well from trying to get him to eat to trying to get him to imitate comfortably and I think this helped a lot. Hope you have success through whatever means. k