Dehydrated

My 6 year old son is 37lbs and 42in tall. When he was about 1yr old we noticed he was getting vomiting colds often, diagnosed by physicians. After 2yrs old I noticed it came about after playing with children for long periods. I asked if it was a sign of dehydration and was told no. After his 3rd bday we had to take him to the ER because we could not wake him, he was so severely dehydrated. In utero and after birth he was diagnosed with hydro nephrosis. He continues to randomly become severely dehydrated almost every other month now. We are at a loss and so are his specialists. Please help us before my son gets so ill its too late.