J.K.
Will he eat table food or does he gag on that as well? I wouldn't bother with Stage 3 food-waste of money. If he will eat table food, I would start giving him the basics, like cut up fruits and soft veggies, cheese, cherrios, yogurt, bread, etc.
Now, if he will NOT eat table food, he may have a sensitivity to certain textures. Worst case scenario he may have a sensory disorder. My son would do the same thing when he was that age. I could not get ANY table food in him besides yogurt. Anything with lumps he would gag on. He ate baby food until he was over 2 years old!! Turns out he indeed had a texture sensitivity and was literally not able to tolerate food textures in his mouth.
If your son continues to gag on food or if he refuses to eat anything but smooth-textured foods, I would definately bring it up at his next doctors visit. Your doc may send him for a "swallow study" to see of there are any physical abnormalities that are preventing him from swallowing.
My son worked with a speech therapist for about year on his sensory issues and his speech. He is now 3 and able to eat other foods, but I am still having a hard time getting him to eat fruits, veggies, and meat.
Good luck with everything. Keep offering him food and see how he reacts. Keep us all posted!