Hydrating Drink

My 9 year old son has had a rough week being sick,he has a history of pneumonia and most times when he catches something from school it's not fun for him or for us.He
also has had gerd since he was a little guy.Anyway earlier in the week he vomitted
numerous times and can't stop coughing.When in the hospital with pneumonia years ago
he developed leg cramps and trouble walking,the doctor did some blood work and mentioned he was low in potassium and gave him an iv bag of potassium which seemed
to cure his leg problem.I have 2 questions.The first one being i read on here i'm
sure about a hydrating drink concoction but forget what the ingredients were.The
second one being what can we give him that can quickly raise his potassium levels.
He has always ate a lot of peanut butter and drinks a lot of orange juice.He is not
a lover of bananas and kiwi and seems to gag when any of these fruits are given to him.I know that he has to go on a mucusless way of eating.I'm sure mucous forming foods are the biggest culprit in his sickness.Any advice?I've taken him a few times
last year to the chiropractor and he mentioned at the time that my son has the same
problem with his iliac joints that i have,not sure if that means anything.

thanks