Milk!

Hi my baby boy is 4 months old and I am worried about the amount of milk he drinks through out the day. I started to give him porridge in the mornings (just a little bit) after 2 hours I will give him milk but he will only have 4 ounces or less. After that he can go on up to 4-5 hours without milk. I get worried and offer him milk but he will only have 2-3 ounces. This is not everyday, sometimes he will happily drink. The maximum he will have in 24 hours is 21-22 ounces. Just wondering if this is enough? He was born at 38 weeks weighing 6.6lbs and everytime we check his weight he seems to be doing well. I also thought about introducing vegetable pures once a day but now I am worried he will completely come off his milk.