Is it normal for my 15 month old son to not want to eat as much all of the sudden ? and to wake up every hour at night crying then going back to sleep ?

These may or may not be related issues. Kids at that age often have spurts of growth, sustained by good feeding followed by periods where they plateau & can seem uninterested. Night waking is a complex issue and would require more information to understand and provide feed back.

A change in his pattern of eating and sleeping should be evaluated. Monitor temperature and breathing, offer a variety of healthy meals and snacks. Allow choices, establish bedtime routine including a story, bath time prior to bedtime. Watch for signs of vomiting and cough.