Hi Rebecca and welcome!
No experience with your exact circumstance, but I do have a DS (age 15 now) who had a severe corn allergy as a young child and a dx of FTT at age 4. He has continued weight issues (he's 5' 9" and maybe 90 lbs. on a good day). His corn allergy caused malabsorption and intestinal damage.
I would suggest having allergy testing done to confirm the corn allergy before you jump through hoops to get his formula changed, though. If he is corn allergic, it's important to be aware of all the places corn can lurk - diapers, facial tissue, medicines, food packaging - and that's an important one because it does not have to be labeled, so even the new formula could contain corn even though it's not on the ingredient list - corn starch is sometimes dusted on the insides of cans, corn oil can be used to coat the insides of boxes, plastic can be made from corn... Most of the -ose ingredients (dextrose, fructose, etc.) can be corn derived. Oh, also IV solutions. When DS was little most had corn derived ingredients (especially dextrose) but lactated ringers was ok. His medical alert bracelet even said 'no dextrose IV - lactated ringers only' because that's something they needed to know right away before/if they took the time to call Medical Alert for more info.
I'm going to split your post from this Welcome topic and make it a thread in the Main section of our board so that others will more apt to see it.