I know you mentioned you have him on 30/18, does this mean you're tried the standard Large puppy food? https://www.purina.com/pro-plan/dogs/dry-dog-food/development-puppy-large-breed-beef-rice-probiotic

Did your current vet discuss why they recommend a grain free?

Typically if you're noticing food allergies, which chicken is one of the most common, your vet may have you do an elimination diet - but that should be happening with care... either you're removing all possible allergens to see if there are improvements and then slowly reintroducing ingredients back individually to identify the problematic protein, or you're removing those ingredients individually. https://vetnutrition.tufts.edu/2022/04/think-your-pet-has-a-food-allergy-eliminating-mistakes-in-elimination-diet-trials/ is a good article from the nutrition experts at Tufts about this process.

Curious if simply bumping the daily amount would help with maintaining the proper weight gain or not if you were to simply use the 28/13

It could be possible to work with a board certified nutritionist... You might consider asking your vet for a referral. Additionally there is https://acvn.org/directory/ (for US)