It turned into one of those "father-son" moments... mostly because Becky didn't want to deal with this particular issue. So I brought Eddie into the bathroom, handed him a bottle of my Avon Black Suede (very manly!) roll-on deodorant, and taught him how to apply it, and told him to put it on every morning when he brushes his teeth.
I told Eddie, "You're old enough now, Eddie, you can start using this deodorant. This is some of Daddy's deodorant, and you can be just like me."
Eddie looked quizzically at the deodorant.
Luke was instantly curious, "What's it do?" he interrupted.
"Just wait, Luke, while I explain it to Eddie," I said.
"So, you take the bottle and you stroke it two times up and down your armpit," I continued, "Just two times is enough." I knew if I didn't say this, he'd use half a bottle on each arm.
Eddie diligently applied the deodorant.
"What's it do?" Luke asked again.
"Come and smell," I told Luke.

Without a trace of shame, and almost proudly, Eddie finally answered Luke's question, "I have to use this deodorant because I have B.U."