Teat size

Ni

My baby is 9 weeks old exclusively formula fed

He is still on the newborn bottle and teat

At 7 weeks old I changed his teat to a size up, being 1 month + size and he hated it so much cried and carried on so we changed back and he was fine

Now again today he has been crying at every feed and not wanting to finish, 4 Oz milk is taking almost an hour with so much struggle

Has anyone changed teats? What were the signs please 😭