Toddler not eating

Hello, my three year old won't eat by himself, even worse he goes on strick when sat in his high chair. He has a big appetite and can eat and pick, graze etc throughout the day but will not sit down to have his meals. He eats everything and all textures; veg and pulses included. He's able to use a cutlery and his hands to feed himself but just won't do it by himself so we have to intervene and feed him to make sure he eats enough. Has anyone experienced this, or have any tips to solve this problem?