15 month old son has had diarrhea (4-8 watery stools per day) for 7 days now. The past 2-3 days he has been sleeping more than usual.?

See your doctor. This is a long time to have diarrhea, and you are right to be concerned about dehydration. Is your child happy and playing when not asleep? If not, see your pediatrician right away. Meanwhile, encourage fluids in small frequent quantity, as well as bananas, rice, applesauce and toast (BRAT anti-diarrheal diet).