Has anyone had problems with the cereal sitting at the bottom of the bottom rather than thickening the formula?

My 5 week old grandson has been put on Neocate but literally chokes 2 hours after his feeding..It's an awful experience for him and his parents..My daughter tried using rice cereal but it didn't thicken the formula it just sat at the bottom of the bottle..We have tried so many thickeners..the doctor didn't object as long as it didn't contain any whey or milk products..He has a cow milk allergy and is truly in a lot of pain plus blood in his stool..How do all of you use the cereal? My daughter would rather use cereal as a thickener rather than using cornstarch which seems to be the only solution for him right now..It's so sad to see....Appreciate any helpful answers..Thank you...