There's no need to add cereal to bottles, not only is it a choking hazard but it's empty calories and bulk they don't need. He should be getting nutrition from formula and, at 9 months, food. If he seems small, check with the doctor, but if the doctor is not concerned and he is following his weight curve, then you shouldn't be either. Some kids are just smaller or drink less. Mine was never a "chunky" baby but she's 64th percentile for weight and 84th for height, she's a lot taller than she seems.
Ok is it better to also give him less purse and more milk ?
Does he eat purées/other food along with milk? If he isn’t, I would introduce it. Might be taking less milk because he’s ready for more texture
Please don’t add anything to his bottle. This is a choking hazard and is not necessary. People are all different shapes and sizes. No two people are the same. If you are worried see a paediatrician but it’s likely there is no issues.