2.5yrs old refuse to eat vegetables and picky on food

My 2.5 yo also started disliking veggie, especially green veggies. In cc, teachers also said he will leave out the veggies and finish other food. But they will encourage him to eat veggies using peer pressure. Sometimes he tried but gags abit. At home, I tried to hide the veggies in his food, it usually works if the veggies are cooked soft enough. I also tried substituting green veggies with other colours of veggies (e.g. purple califlower).