L.M.
Hi, S.! I agree with the other two responses... do not let this go! If your pediatrician and any other doctor don't know what is wrong with him-- get him to a Children's Hospital for evaluation. If the first doctor you see there doesn't help, DEMAND another.
Surprisingly, we moms often have to push to get the medical care we need for our kids. Don't worry about seeming pushy. This is your child's health, and at 20 months old, he needs YOU to be his voice and his champion.
This doesn't sound like normal behavior... and I'm not trying to scare you (it could be something very simple), but you need to find an answer.
Good luck, and I hope everything turns out for your little boy.