Does anyone know what this is?

He has these bumps on his arms. He’s had them since he was a baby. (He’s 2 now). We believed it was just an allergic reaction. But it only appears on his arms. Every time we bring it up to the doctor they just kinda blow us off. Lately they’ve been looking worse. We have another appointment scheduled with his pediatrician, but it’s about a month away. So just looking for advice here in the meantime. It’s not painful, or itchy. He seems unaffected by it. But it still worries me.