I might stop thickening and see what happens?
is he on meds? the meds should stop the pain so even if he is spitting up, it should not hurt him. if he is having trouble gaining weight and that’s why you need the thickening, then I would go to a nutritionist/dietician to find out how to enrich his formula w/ extra calories, etc.
if you are thickening bcse you are worried that he is getting thinner liquid into his lungs (aspiration), then I would ask for a Swallow Study which will tell exactly the mechanics of how he’s swallowing and then if he shows that he does need thickening they will tell you exactly what consistancy (using terms like ‘nectar’ or ‘honey’) and help you get this consistancy w/ thickening agents like Simply Thick or Thick It.