Why my son don't like to drink milk? If he drinks, he tends to throw out after burping.?

My son is 3mths old now. For the past one week, he refused to drink milk. However, he loves to drink water. When feed him with milk, he stop sucking but when drinking plain water, he can finish abt 60ml each time. What should I do? His feeding is abt 850ml per day but now it drops to abt 600ml per day. Also, when we burp him after feeding, he will throw out all the milk. Out of 5 feedings, he throws out 3 times. Sigh!