Re: not nursing much and weight loss 3 1/2 year old
I would imagine dropping several nursing sessions without replacing it with increasing solids would be a pretty good explanation of the loss. That said, my son is the same age and has a lot of the same allergies, but hasn't nursed since he was 9 mos old, and his weight tends to fluctuate pretty widely at times, too.
If you can't get him to nurse more, I'd try focusing on solids. Can you try getting coconut milk into him in other ways? Smoothies (chocolate coconut milk+banana=delicious), in cereal (my little guy is a big fan of cinnamon apple rice chex), in sauces or gravies (I put a little in pasta with his fake butter and some herbs- he loves it). Look at the higher fat nutrient dense foods like avocado, beans, whole grains,flax seed is a really good addition to almost anything, ground up and sprinkled on top.
Have you seen a dietician to help you make sure you're getting him a well-rounded diet? Your ped might be able to refer you to one and get it covered by your insurance (ours did).
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