What would cause excessive sweating in a 5year old boy in the winter, only at night when hes sleeping?

Night seats. Excessive night sweating by your child requires him to be seen please.
Commonly. I have seen this a lot with snoring and apnea but also related to parasomnia. Some people only notice it in winter or springtime per se because of nasal congestion being increased at that time due to dryness or allergies.