My son sweats alot in sleep he can wet up a whole pillow is this dangerous?

Depends: Its important to make sure the sleeping environment is comfortable and cool. Im guessing in Mississippi it may be too warm for comfort, so adjust the sleeping arrangements would be a good first step. Dehydration can be a serious problem for a child if severe, but its hard to tell without more information here, if you have doubts, see a pediatrician and discuss in detail

Need blood work: You should get his thyroid checked as well as his adrenal hormones , excess of both can lead due to increased sweating