Oatmeal to thicken milk for reflux?

Ladies, my LO is ~4 weeks old and was born 6 weeks early. He has awful acid reflux / colic so the pediatrician recommended thickening my breast milk with cereal. I chose oatmeal since rice cereal has arsenic.. we’d been using preemie and slow flow nipples whenever we put him to bottle before — but now, the milk is too thick to pass thru. Any suggestions? I have dr brown’s bottles, como tomo, and avent. What level nipple should i use? The y cut for como tomo, for example, was way too fast. He drank it in 4 min and spit all of it up. Thanks!!