Husband has lump

My husband noticed a small lump under his armpit for a few months now. Recently he checked and it’s still there and bigger and very hard. He says it hurts him. He pushes under his arm pit all day. I touched it and it feels like a hard round rock. There is no white head at center to puncture. It’s like a small ball under his skin under his armpit. It’s not squishy or leaking. I hope it’s just a under skin boil but I’ve seen those before and this doesn’t look like a bump or skin bump. I’m so nervous. We are scheduling a doctors visit soon.