I think Hotl gave you some great advice. Maybe try some sticks of avocado, it's really soft and prob a good start. we never really did alot of mash and purée, mostly steamed veg sticks etc. Mango is another great one. Have you tried giving him foods and letting him explore? Maybe like Hotl says there could be more to it, a physical reason why he still gags. But perhaps if you let him try being in control of what he puts in his mouth he might start to eat some solid foods. That must be tough for you, good luck.