My bf get sores on his skin that start off as blisters and its on on his upper extremity what could this be?

Once, more? 1 arm? 2? Once? Could be shingles, flare of prior chicken pox infection. Can't get it if vaccinated, without history of infection. Not both arms. Could also be bacterial infection of skin (impetigo). Multple? Could be manifestation of celiac sprue: dermatitis herpetiformis. Other causes are possible too, e.G reaction to sun, genetic metabolic problems-less common. See md, preferably during outbreak.