It's unbelievable, but my roommate washes his clothes with all purpose dish detergent.
That's really strange, why doesn't he use laundry detergent?
I don't know, I've suggested it to him but he doesn't want to use it.
All you can do is make helpful suggestions and hope he follows them. You also don't need to stop him from doing things, he is a big boy.
I know he's an adult but I hate seeing people make mistakes that could be easily corrected.
I'm sure it's frustrating but every once in awhile you run into someone that wants to do their own thing. It doesn't matter how many times you say something, they just want to be unique and different. Try not to get frustrated and just accept your roommate for who is.