You can try giving him more formula to see if he prefers fm. It might be the food you intake that causes the bm to taste different. Probably you can note down of the food you took and see if he’s ok with the bm. If he’s pooping and peeing well with a consistent weight gain I think it’s ok. Anyway you did added in bm in his cereal so i think it’s fine. If you feels that his milk intake is not enough, you can just give him fm (if he rejects bm) in his next feed.