K.H.
I would try some very soft finger/table foods for him. Stage 3 foods are a really odd texture to me, and I'm not surprised they gag on them. They have that smoothness of the purees, and then weird chunks thrown in, which can catch them off guard, when they are just expecting the smooth. He's at a good age to start learning to pick up foods, and you can start with some really easy foods, like tiny chunks of banana, soft cooked apple, avocado, shredded cheeses, toast, etc.
Good luck!