D4Ever - I hear you. I am the only woman in this household and the only frequent rider. I gave up trying to teach position a long time ago. The guys are out to have a good time once in a while and that's it.
MDH doesn't have this issue because he wears tightie-whities *all the time* <sigh>. At least they're not actually white. Plus, his current horse is just barely ready for MDH to trot on her.
Our oldest son refuses to wear supportive clothing, the youngest loves the body-hugging spandex boxers for regular wear. So, the oldest's solutions? Well, he found a way to sit that worked for him on a particular saddle. It's a very bad-western-movie-cowboy-position, but it works for him and he has held his place even when his horse spooked. At a trot, if he's riding the little mare, who is quite rough, he takes just a bit of weight off the saddle bit by putting weight into his stirrups. Ya, it's not right, but it works for the once-every-two-months ride. A canter isn't usually an issue.
If you have a choice of saddles, let him try other ones. That also makes a difference for both MDH and our son.
As much as everyone loves to tell you that he needs to have proper position, I hear you that it's not in the cards. Take what you can get! I agree.