Given creatine is present in several foods, do you have an intolerance to those as well as creatine in supplement form? Curious if it's a simple allergy or the dosage.
Creatine breaks when cooked, so for the most part he's not gonna get too much from eating a steak or whatever. Taking 5g/day of pure creatine to satiate your stores might be too much for this particular individual. Anyway, the parent produces its own creatine as well!