Keep offering food, but don't push and don't worry. Keep up with the formula until he starts to develop interest and loses the gag reflex.
My firstborn barely ate solids at all until he was past 18 months, and he had toddler formula until he was 2. He had a bit of a texture/sensory issue and he remained a selective eater until he was about 12, when he started to branch out. He's nearly 18 now and much more food-adventurous.
The main thing is to not create an issue or start a food battle. Neither of you needs that.
Regarding a comment below as a point of clarification - the poster is mistaking the ingredient 'corn syrup solids' (dextrose, about 75% less sweet than cane sugar) with high-fructose corn syrup (fructose, and sweeter than sugar).