When my DS was 2.5 his diet was so limited I took him to the feeding clinic (Salt, physio, Nutritionist etc) He does have some SN including sensory issues (and stubborn as a mule issues ) He ate beautifully until he was 18mth-2yrs, so I was tearing my hair out as he only ate baked spud mashed with butter and cheese, pizza, goats milk, fromage frais, bananas, bread and butter and only a few other things. He was really skinny.
He has now just turned 4, he eats loads of things! Not everything you give him, apart from at nursery school where he cleans his plate the 2 lunchtimes he is there!
I started by putting brocolli on his plate everyday with a food he would eat, I asked him to try it everyday and one day he just did. It wasn't long before he started saying more brocolli! I never told him off for leaving food and always praised him for trying new things.
I started letting him cook with me, he then tried stilton (yum), mushrooms (yuck), cake, icing, biscuits, chips, we make homemade fish fingers with salmon he loves helping and eating when they are cooked.
I just put things on his plate until he become familiar with them and this has worked for cucumber, lettuce, roast pots, yourkshire pudding (he can now eat 4 at one meal!) pancakes (ditto) etc.
At the weekend he had Covent Garden tomato and marscapone soup for lunch (he said yum and asked for more), with bread and cheese. I started him on homemade butternut squash as it is so sweet and can be made thick and smooth - so not a challenging texture. He will now eat roast squash with roast dinner.
My thought is to make every mouthful count.
I used to add cream and butter to mash to get some weight on him. I put 2 eggs in yorkshires for extra protein. Pancakes I do little fat ones (3 eggs, 3 tablespoons of full fat natural yoghurt, 1 tablespoon of plain flour) for protein and calcium and he has these with sliced banana and sugar. I make my own fish fingers with organic salmon dipped in egg, rolled in crushed cornflakes and fried.
So if will only eat pizza/chicken dippers make your own with the best quality ingredients you can afford.
Good luck