Booster and prone to a allergic reaction

‼️ this is not an anti-vaxx/pro-vaxx question ‼️

My brother got 2 doses of Moderna and w his second one he had a pretty bad reaction. Fever, diarrhea, vomiting, rash all over his body... they prescribed him w Benadryl and he was fine within a week.

Nobody else in our family got any side effects, and we’re all fully vaccinated. My fiancé and I got our boosters on Monday and we had some very mild flu-like symptoms for 2 days. However we got Pfizer, and I’m thinking maybe it’s better? Now my question is, my brother wants to get his booster, but we’re concerned about his history of a allergic reaction. Any tips? Suggestions?