My son is refusing to eat his dinner

My son is 3 years old and has down syndrome. Unfortunately he has a 
Iimited range of foods, for lunch this is soup and beans not together. He wont eat sandwiches at all, it could be a texture thing. 

His range of main meals seems to diminishing as well mainly pasta and cottage pie. 
He will try a few mouthfulls and then shake his head and ask for yoghurt. Im not sure if he just being fussy or its something more serious. 

We are starting to worry that he isnt getting enough food and it will make him ill, its been going on for months. Does anyone please have any advice on what we can do.