I am usually a stickler for the rules but started DD on cows milk drinks at 11 months. DD was slowly weaning off the boob and I didn't want to mess around with formula. It is recommended not to use cows milk as a formula/breast replacement before 12 months, especially if you have any family history of reactions to dairy. Having said that, in this case I was comforatable going against the recommendations. I started introducing milk into DD's diet through breakfast (cereal) from 9 months so knew she wasn't reacting to it. She was only having about half a sippy cup worth of cows milk during the day until 3 weeks ago when I fully weaned her off the boob and she increased to 1.5 sippy cups full of cows milk a day.
If your bub is going well on formula, then I'd stick it out a couple more months - just to be safe. Continue cooking with milk and offering dairy, thats a good thing to be doing. Maybe as he gets closer to 12 months you can try offering a small cows milk drink during the day to see what he thinks.
What ever you choose, best of luck.
DD is 3yr 8 months - DS is 6 months