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My children have had some of my Apple Cider Vinegar drink perhaps as young as 9 months old, but only sips here and there just because they wanted to try what mama was drinking. I don't know that I would give it to a 6 month old...thought it would be safer than most over the counter meds.
For chest congestion, you could try garlic salve. We have found it to work very well. Here is a link to the recipe. I would use 6 cloves garlic and not 8 for a 6 month old. And using this may clear up all of the congestion. Your baby will smell like gourmet pizza!
http://newlifeonahomestead.com/2013/01/cure-a-cough-naturally/
You could also try taking milk out of your diet, if you consume it, and see if that helps him. I found that consuming dairy products while breastfeeding did not agree with most of my children. (I usually did not have access to raw dairy, so that might have been different.)
Let us know how he is doing!
~Mama to Many~
ACV for Infants: Safety & Usage
★★★★★
But I am sure that ACV would not be harmful as its a food, but I am not a doctor. Just a mother :) Also Chiropractors are great, if you have ever taken your little one to one. They can adjust their ears and prevents lots of illnesses as mine told me many illnesses are connected to our ears.