Cue how many times has he had it now? I don't want to scare you but my friends son has had a rare reaction to predmix and now goes in and out of coma. Please take him to hospital.
How old is he? My dd had her tonsils out at 14 months for similar reasons. Rush the feed test and thank goodness for clear test for CP. Babies are so scary. My only other suggestion is try thickner in his fluids?