Just be sure he keeps eating gluten daily for the testing at least 2 slices of bread. The blood testing requires this for 8-12 weeks to be accurate and the scope with biopsies from the GI will need this for 2 weeks.
It is not in ALL broths, there are a few that do not have the yeast extract oddly very few and it pissed me off every time I go to the store. I oddly found a Swanson Unsalted Cooking Stock this year...quite bland lacking any seasonings, flavorings etc. I used to use Pacific Foods STOCK (The broth has yeast extract), or this one Kitchen Basics (The chicken bone broth was the only one). I also have some supplement broths from Jarrow foods in powder form that are safe and give no issues.
If he had a low positive on the TTG IgA and he has a low iron level, you should see a Gastroenterologist for further evaluation. Anemia (low iron) is common in celiacs. I would not wait three more months. Why make your son suffer? Get another opinion is my best advice.