I have to agree with grandma. Variety is the key. He also may have sensory issues. That would be something an OT can help with. In the meantime, try smoothies. Just blend part of a banana, and one or two other fruits and veggies. You can get a variety of protein powder that is not soy or dairy. There is pea protein and hemp protein and veggie protein. Try Vega brand, they are great. When feeing veggies you can give them raw or steam them lightly. Some moms have had success with then cutting them into various shapes with cookie cutters. Try other grains: quinoa, barley, millet, teff. Dive into beans: lentils, black , great northern, pinto, kidney, aduki and so many more. I'd suggest going to MOM's in Bowie, I think that is the closest one to you and looking through the store for various foods that fit outside his allergy zone.
Good luck,
L.