Keflex is a semi synthetic, first generation cephalosporin antibiotic. Bactrim is a synergistic fixed combination of sulfamethoxazole (an intermediate-acting antibacterial sulfonamide), and trimethoprim; both sulfamethoxazole and trimethoprim are synthetic folate-antagonist anti-infectives.
Did you give you a diagnosis for the boils? If so what was it. Then I can tell you why he might have. He seems to be trying to make sure he kills both gram positive and negative bacteria.