Only wants oatmeal

My son is 12.5 months old and doesn’t want to eat anything except oatmeal and his toddler (12+) formula. Also will eat some baby cookies and snacks but I don’t count it as food.

Rarely he will bite tomatoe, pear and eat banana.. he used to eat almost normal up to 11.5 months and now suddenly stopped.

Like today he ate 2 tsp yogurt. Few sips smoothie. I tried to give him veggies, chicken, fruit, he just turns his head right away and makes disgusted face.. and won’t open mouth to eat. Sometimes he tries a bite and then again makes so disgusted face and shrugs. Tries to spit out. Sometimes eats 3 small slices of zucchini of from our pizza.

His weight is normal. But just little bit below average.

Anyone has similar experience? What to do?

After 4-5 hrs I give him anything he won’t eat unless oatmeal (I mix in fruit and formula) or formula.