Tell him deodorant doesn’t block sweat but anti-perspirant does. There’s a big difference.

Deodorant kills/lessens the odour causing bacteria, so if he’s worried about pheromones or some junk he read/heard online he’ll be fine with deodorant.

Also maybe mention worrying about whether things are manly or not is not particularly manly, but I too went through a phase of trying to figure out what that meant for me.

Probably the most cringe was my teenage attempts to grow a moustache and beard. It did not turn out well. I got an intervention by my boss handing me a razor, escorting me to the washroom, and telling me not to start my shift until I’d shaved.