That really is interesting. I can't believe I completely missed this.

I think Dean might not call Castiel by his name in the first few episodes because that would in some way make him a person in contrast to just another supernatural being.

And after that... I tend to call him "Castiel" when I refer to the otherworldly angel and "Cas" when I'm talking about the side of him that has doubts and feelings. Maybe Dean, subconsciously, makes a similar distinction but he only ever talks to Cas as soon as he has figured out that this side of the angel exists.