there are many issues here. THE first one is this 'friendship' between father and son. although I consider normal an open relationship between you two, it's not very normal to shower with your son at age 8 cause it is the beginning of his sexuality, and it can sexualize the relation father|son. ANYWAY it seems you have that kind of relation that wont lead to an 'edipus' [in this case an ELECTRA] complex with the son in love with his father.
probably he was just interested in seeing some cock. apart from the rude way of saying it, he PROBABLY wants to see how he father looks, and compare his own with you.

JUST one question, and maybe it's because I'm very old fashion [even being a gay man], but how can a father be 'openly' 70% gay|30% straight and maintain the 'typical' father|son relationship?
MAYBE due to the fact [and I'm not judging just speculating] of your apparently 'open' sexuality, your own son can have some doubts about himself and questions about his body and sexual orientation.
TELL us how it goes. and my advice to you would be JUST to talk to him, cause dialog is ALWAYS the best answer!

CHEER and good luck!