How many solid meals is your son taking? And how much of solids?
My son now at 13mths only drinks 500ml of milk a day with 2 solid feeds. 1 solid feed is 2 chinese bowl of porridge and 1 solid feed is 1.5 chinese bowl of cereal.
Before I increased his solids amount, he can take up to 800ml a day.
Maybe your son is thirsty? Have you tried introducing water or diluted juice?
Usually on the tin, there's a "recommended" feeding table which tells you roughly how much of milk that age group is drinking. You can use that as a guide.