I sympathise, but from the age of about 12 onwards I used to expose myself (making it look accidental) to our female neighbours. Luckily they never complained, and before I was 15 I grew out of it.
It looks like you have 2 choices.
Stay where you are because you are comfy and taken care of and just know things WILL NOT CHANGE
Or
Fucking leave with your dignity.
Clearly you are with someone who could give a shit about your feelings or he would have and SHOULD have put a stop to the grandson's bullshit.
My guess is that you will stay until things self destruct.
Good luck
Either he's a touch off or he's advertising that his penis is available. Gthe lad into counseling. Get him a date. Send him off to school. If none of that works ... hit it with a strap. Bet then the little pervert will put and keep it out of your sight.
So Brian deviates from "normal" parameters. Nowadays, parameters regarding all topics are subject to change almost daily. Brian is not getting into any troubling issues with the law or neighbors. Furthermore, people are more tolerant in the new century we have as well. You on the other hand, appear to be reacting with far too much emotion regarding Brian. You are also being Sexist. Look, if this were a daughter of your partner with bad habits, I'm certain you'd be letting it slide due to you sympathizing with your own gender. You're practicing sexism and I advise you to keep an open mind regarding someone else's offspring. I'm sure you are no saint.