We went in ounces (so say 4 oz formula, 1oz milk) and increased the milk over the course of a couple of weeks. But that was 6 months ago, you could probably go faster now. If he has been having regular formula (I.e. not CMPA milk) he's probably not allergic, because most regular formula is derived from milk
I went straight in with cows warm milk, 7oz morning and 7oz before bed. She took to it and didn't realise the difference. Good luck x
I would do half milk / half formula and see how he gets on. Then if he’s fine with that transition to full milk as at this age they get most of their nutrients from food. I will say they tend to get a bit constipated when they transition to full milk. So make sure he has enough water in the day too, I’ve even thinned milk with water before when my child was particularly constipated! X