Formula for breastfed baby?

My baby is 9 months and has never had formula, but I think my milk supply is sadly decreasing so I may need to supplement with formula soon. We recently tried Enfamil’s Enspire formula mixed in his oatmeal and my poor baby broke out in red hives and bumps on his face and neck. Luckily it went away within a half hour. Any suggestions on which formula to try?