Need help for my son.

Hi,
My 12 year old suffers from terrible migraines and we gave him a small dose of tylenol. Right after he took it, he started complaining of sore muscles in his thighs (not achey or cramps just sore when you touch them or move). We took him to the ER and they did no tests on him. Just said the muscle pains could be post-viral. He did have a stomach virus, but was over it for a couple days before the pain in his muscles started. Does this sound like he has liver damage? I'm really scared.
BTW,
He weighs 75 pounds and I underdosed him at 2 teaspoons I believe. He doesn't take it often and the last time he took it was probably 3-4 weeks ago. I have tried cayenne pepper and salt substitute for his migraines before, and they don't work as well as we like. Tweet