I would start weening him off the formula now, some babies don't like the taste of milk, but i went cold turkey with 3 out of my 5 kids. I put it in their bottle and warmed it in the microwave for about 15 seconds and shook it up and had them taste it and my youngest son who is 18 months just switched to whole milk from stage 2 formula, just because i needed to feed him a higher calorie formula because he has a CHD but i went cold turkey with him and he loves the milk. Try a couple bottle of milk only to see if he will take it, he might push it away at first but i sang a little song to distract him and now no problems, thank god lol because i was paying 28 dollars a week for his formula!!! But if he doesn't like it then just slowly start mixing some with his formula the first week with decreasing the formula and by the end of the week he should be on just the milk. Good Luck!
S. mom to 5 amazing children 17yrs,9yrs,7yrs,3yrs and 18months check out my sons journey at www.liamlockhart.com