are you a female? thats the only way I can think this would be embarassing.
I would simply say to my brother, "Dude put your f'n boner away."
If it was karky, he'd prob say
''hey can i put that away for ya''
I usually tuck mine on the elastic part of my boxers so its facing up hehe, if someone lifted my shirt up tho theyd just see him looking at em.