2 month old pooping solid

My son is 9 weeks old and he has been having a tough couple of weeks. He was on similac pro advanced for 7 weeks, and had suffered from a lot of gas. His poops were normal throughout this time. I spoke with the doctor and he switched us to similac total comfort. With that, his poops were explosive and very seedy from undigested protein.

After about a week, he started having really bad reflux and was spitting up/not eating and crying through feedings. The pediatrician had us switch again to the similac alimentum, and to add 1 tsp of oatmeal per ounce of formula. He typically eats 5 ounces at a time. We also started giving him Zantac for his reflux He’s 13.7 pounds. After about 4 days of that, he got his 2 month vaccinations. Since then, he is only part

little turds and they are solid and similar to what adult poop looks like. It is brown in color and he seems to be constipated. Since there were so many changes I’m not sure what is causing for his poop to be so very different and I’m worried because he is still not eating as much as he was before.. any one have any similar experiences?