Iv had the same problem! My daughter is 2 and from 8 weeks was on Nutramigen due to a milk allergy. We tried her with cows milk at 13 months and she took a reaction to it and were advised to try again after her 2nd bday. (We havent got round to that yet).
I do know what you are going through but as at around 20 months we tried to introduce soya milk, so a new taste. Caitlin wont take it at all. In fact we got rid of her btls shortly before this and now Caitlin wont take her formula in a cup or soya milk so is getting none which isnt ideal.
Some people suggested adding Nesquick milkshake powder to the milk? To make it taste appealing. You then gradually reduce the powder until eventually they taking plain milk. You could try that? It didnt work for us but