1 month Spitting up curdled milk

My son was on Similac advanced and getting breastfed also. I noticed that he kept spitting up curdled milk over and over and would be very fussy so I talked to his doctor concerning reflux and she said it was normal and that he's gaining the weight he needs to gain so I shouldn't worry. Okay same thing still happening so I decided to switch to Similac sensitive... I noticed that he would spit up but it won't be curdled.. It would just be a smooth texture. Today is the third day of Similac sensitive and all day he's been spitting up...After I nurse him and after formula close to all of what he drank. Along with that very very fussy as if he's in pain and I noticed that the milk is curdled again. Ugh I'm so sad I wish I knew what's going on.. Anyone ever go through this.. And should I switch doctors and get a second opinion? I honestly think he has reflux but I'm no doctor. Smh