This might be an awkward conversation starter, but you could be like “i’m curious, what’s it like showering once a week? I’ve been raised to do so everyday without fail, so i’m womdering how you feel about it” or something of the sort. It won’t be calling him out per se, but it could start a discussion, and you’d go off from there. Maybe if you say “I start feeling really dirty if I don’t take my daily shower”, he could ask if he stinks or something hahaha
My brother in law eats like a wild savage. The guy doesn’t close his mouth while eating and slurps up the food. My wife eats like a normal human, thankfully.
Although pointing out at the very moment of occurrence, specifically laying down the pros and cons.
You have observed the spectrum of ill-manners, perhaps you gotta straightforward discuss the grooming part in everyone’s life, irrespective of the time frame and how it’s a life-long journey.
Just tell ‘em. The term rude gets on my nerves. I’d want to know if I was not being pleasant to be around. And then practice what you preach. It should work.