H.B.
Hi M.! My son, Ethan, did the same thing at that age. At 8 months he refused to take a bottle and would only use a sippy cup and by 10 months he too did not want formula. I began giving him regular milk a little at a time and eventually got to a full cup(within a couple of weeks). I was worried about the nutrients as well so I put infant vitamin drops in his milk and he drank it fine. He never had any problems. I think he was taking in enough "big boy" food plus the vitamins and that was enough nutritionally. Also, sometimes kids just prefer real food because it tastes much better than formula. Plus, your son is growing fine so I wouldn't worry about it. I figured that back in the day before formula(which wasn't that long ago) regular milk was perfectly acceptable and our grandparents and parents came out just fine!