Have a speech therapy evaluation. Our last foster did not know how to eat foods properly because he had been fed chicken nuggets and hot dogs starting at 6months. Mom told me that she thought he was "such a smart baby" that he could handle foods. As a result he developed poor oral habits and could not manage certain foods in his mouth. Occupational therapy should also be consulted to assess for sensory issues. My son developed a food aversion due to sensory issues. There are a few textures that he will not eat even after 5 years of therapy, but eats enough variety that he gets proper nutrition.
Duocal. It mixes with pretty much everything and is a great way to add calories.
There are some wonderful books and resources about food chaining. We used food chaining as a way to get my 100% orally adverse, tube fed, failure to thrive son to eat. It works well.