I would guess right off the bat that Catherine would call him Ray. After all, she usually called Grissom, Gil, and many times would call Nick, Nicky. I'm guessing for most of the CSIs, while it might be Dr Langston for a little bit, it would turn a bit more informal later on. I don't think the "Doctor" prefix would last long. Grissom was a "Doctor", but it was mainly only used by those outside of the lab. I think the lab rats would probably use either Dr Langston or Dr Ray for a little bit as well. They can't just use "Doc", because we already have Doc Robbins.