When my son was diagnosed with Celiac Disease in 1976 as soon as he was weaned, his doctor insisted that we feed him only Nutramigen it was the only hyperallergenic with complete nutrition.  Enfamil and Similac were not acceptable if we wanted him to get healthy.  For one thing they had no  choline back then. Given the lawsuits against them now, he was right. He spent about 6 months on the Nutramigen formula exclusively and weaned off of that to a gluten free diet and thrived. He also predicted that by kindergarden the teacher would beg us to put him on Ritalin, which he took all the way through High School.  

Check the label and tell us what kind of B12 is in your gummies.   If it's Cyanocobalamin, switch to a methylated (active) form of B12.   Some of us need to take the active form of B12 because our bodies have problems turning other forms of B12 into the active methylated form due to MTHFR genetic variations.   Take a B Complex supplement, too, because B12 needs the other B vitamins to function properly.

Hi! I've been gluten free for years. No surgery,  but have dumping syndrome symptoms,  particularly if I've eaten a lot of FODMAP foods, or xanthan gum, or any other gums that are in many gluten free foods. You may want to stop eating foods with gums and see if that helps!

@Zoe26, Has he been checked for vitamin deficiencies?    Is he pulling up to standing position?  Does he crawl? I ask because delayed development can be associated with nutritional deficiencies.   Having a hoarse low voice is symptomatic of Thiamine deficiency in children and adults.  Complete loss of voice is possible, too.   It's horrible you and your son had to wait so long for an appointment.

I found this article very interesting... Carrier frequency of HLA-DQB1*02 allele in patients affected with celiac disease: A systematic review assessing the potential rationale of a targeted allelic genotyping as a first-line screening https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7109277/ Not a doctor.