You have to pick a lane: Send pics and show affection with clear caveats, or find peace with the complicated dynamics you’re helping foster. That means the only way to make it clear that you aren’t interested, that you’re just having some digital fun or whatever it is you’re doing (what are you actually doing?), is to say so explicitly. There’s nothing wrong with sharing nudes (provided the recipient’s consent), but you should understand that along with your muff shots, you’re sending a message of potential sexual interest and/or availability. I’d be a hypocrite if I advised you not to express your sexuality. It doesn’t sound like you are trying at all.
I don’t mean to interrupt your merry, nearly nude traipse through life, but a good way to set boundaries is to actually set them. But I also don’t want him to try to come on to me, because rejecting him in that scenario would also be awkward. How can I keep clear boundaries without hurting his feelings? Flat-out telling him I’m not attracted to him seems like it will kill the vibe.