Toddler barely drinking

My toddler has a runny nose but most concerning to me is he’s barely drinking. This boy usually wants 5 cups of milk at night and tonight he didn’t even finish one after school. (I’m not sure how much he had to drink at school.) I’m going to make sure he goes potty at least every 8 hours but what else can I do? When my son gets sick , he gets SICK. So I’m nervous this is the start of something.