I don't want to get used to my messy roommate
Why don't you like mess?
I just don't like it all.
It is understandable to not like messes. It is not good for you and could be a health hazard. Talk to them about it so they can start picking up for themselves.
What if I don't like my roommate but I don't want to say anything about it? How can I get him to change?
You do have to say something, you cannot put up with his messes. Living in a dirty area can be unhealthy and draw rats and cockroaches to your place.