wishing you a R"S

Allergist is probably the best choice for 1st stop on your list.
I'm sure you have tried pretty much everything, neti pot, himalya salt, xylitol, are all good at helping clear the sinuses, help keep bacteria free, reduce inflamation etc; Something called Umcka made by natures way is really good at reducing mucous and I recently read that is helping people with pollen and other allergies too.
Garlic is good in general and SA for the immune system and even if you can stand it...sniffing some is supposed to totally clear up sinuses....I have'nt tried that one myself yet.
Assuming your ds is also eating solids you could try adding good fats and calories to what he eats. cream, coconut milk/cream YOu can add to almost anything, smoothies, scrambled eggs , applesauce, mashed banas etc;
maybe adding fish oil and probiotics, he might also be allergic /intolerent to some foods.
have you had him tested for gluten intolerence? that is a bigf one in babies /toddlers who have difficulty gaining/thriving etc;
hope it all works out for you, that's a lot to go through.
btw, that's so great that he just picked up nursing when you got home

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