Severe Reflux

My son has severe reflux and his GI specialist wants us to add cereal to his bottles to weight down the formula and hopefully decrease the amount of medication he is on. Those of you that have gone through something similar - what bottle/nipple/brand of cereal worked for you? We can’t seem to stop the cereal from clumping and blocking the nipple opening. We are only using 1-3 tsp in his 4 ounce bottle.