dehydrated?

DS is sick, he was throwing up yesterday, and has had a fever of 102. Motrin brings the fever down until it wears off, and it is right back up. I have been trying to get him to drink water, I am afraid milk will upset his belly again. I changed his diaper at 10:15 this morning, and now it is almost 2 and the diaper is completely dry. That isn't normal for him. When do I worry about him getting dehydrated? He hasn't vomited since about 5 lastnight, but still has the fever, and I can tell he doesn't feel well.