totally normal. mix some instant breakfast with his milk for vitamins if he likes chocolate or vanilla and buy whole milk. all kids go through it.

my daughter did that for a bit and we talked to a nutritionist. She said to limit her to 2 cups of milk a day, the rest water. When shes hungry she will eat. Sometimes little ones just dont eat, and it shouldnt be a cause of concern unless they drop a tremendous amount of weight or are extremely small to begin with.