My daughter's boyfriend had issues flushing the toilet

My daughter is in High School. She's 18 along with her bf. He's a nice kid who treats her right. Before I met him my daughter did explain his home life to me. His mom doesn't clean at all. She leaves trash everywhere and gets animals she doesn't need and lets them use the bathroom all over the house. They don't have a washer and dryer and rarely have hot water. Because of all this him and his siblings basically have to live in filth.

When I first met him he was extremely polite. I did notice when I went to the bathroom after him he didn't flush. I didn't think much of it because I really had to pee but it happened several other times he came over. So the last time I asked if I could talk to him in private and we went in the he hallway. I told him "Honey, you forgot to flush the toilet". He looked confused and said he only peed. I told him you always flush the toilet after you use the bathroom to have good manners. He looked really embarrassed and said oh and started apologizing. I told him it's okay and not to apologize. I also let him know it's okay if he has trouble remember just to try his best to remember to flush. Before he left he went to apologize to me again and I told him he didn't need to keep apologizing. The reason I'm talking about this is because I know a lot of people would rather embarrass someone for not having the best clean habits or manners, but don't stop to think maybe they were never taught. He grew up differently then me so instead of humiliating him I wanted to help him. He's a good kid and treats my daughter well. Plus I know he's already embarrassed by how he lives. I allow him to wash clothes at my house. If he isn't smelling the best I offer him a shower and let him wear my son's clothes so he can wash his. We all have different back stories. Why judge when we can help!

Edit: I do understand some people do the if it's yellow let me mellow thing. What I had meant by manners is to not do it at everyone house unless you know that's something they do. I did tell him if that's what they do at his house that's fine but if he's at someone else's house unless you know for sure that's what they do flush to be sure just in case.