They make cereal nipples with a Y cut in them for that purpose. I used it with my first and plan on it with my next one. I have the Dr Brown bottles with those nipples.
My son also had severe acid reflux and we tried all kinds of things to help him with this. I can't really recommend a certain nipple, but I know that makes a formula that has the pink label called Enfamil AR. It has rice starch added to the formula to make it thicker and this did help our son for quite a while. It came out of the nipple okay and didn't require us to add rice cereal. Just as a side note, our ped had put our son on Zantac but also on Raglan. He told us the Zantac only changed the pH acidity of the stomach so that what was refluxed did not burn the esophagus or throat. The Raglan was used to move the food out of the stomach faster so that there was less food to spit up. I know we all hate having to give our kids so much medicine when they are so young but I wanted to let you know about it in case you would like to look into it. I hope this helps some. Good luck.