You probably need to 'wean' him onto it, to get him used to the taste. When I changed Harry from one formula to another, and then from formula to cows milk, I mixed them. I started off with having 150mls of the old (formula) and 50mls of the new (in our case cows milk). Then gradually added a bit more of the new and a bit less of the old - so 100 and 100, then 50 and 150, then just 200 of the new. It took a couple of weeks, but mixing the new with the old is totally the way to go