My son isn't eating much. How long does this last?

You sound like me! My kids have stages when they hardly eat and they like drinking a lot too. I get really frustrated if one of them does this but I try to give them their food anyway. If they don't want it I tend to just leave them as they will eat when they are hungry. Try to avoid giving him a lot of drinks because that fills their bellies up so when it comes to food time they don't want to eat a lot. I used to take them to the doctor when they did this but there's nothing they can do you just have to ride it out.