My kids go through weird phases all the time. Right now my daughter is transitioning to regular cups from straw cups and while she loves it, she's not drinking nearly enough. With her, I just remind her to drink and make her stop playing. But with a little one things like that are harder. How is the transition to sippy cups going? Have you found one that he likes yet? My son gave up his bottles completely when he found out he like straw cups better. The issue with not drinking is not only about the nutrition he's missing from not drinking formula, but also from the water his body isn't getting. Staying hydrated is very important. Big fluctuations in fluid intake can cause problems with electrolyte levels which lead to other problems. 4-5 days is a long time to have significantly decreased fluid intake. I know babies and toddlers are good at figuring out what their body needs and most of the time, listening to their signals is the best thing to do. But, this is a time when you have to intervene. Do what you can to get him extra fluids in his foods. If he's still eating purees, thin them out with some water and do the same with his cereal. If he's eating some table foods give him ones that have a high water content like watermelon.