Flirt with him. You could say something like "Hang on, I'm about to hop in the shower... ;)" with a winking smiley.
And then text him when you get out and tell him you just got out and you're drying off, etc. Subtle things. If he plays along, it will be a lot easier because you will be playing off of each other.
say that your horny and that you wish that he was there to help you get over your hornyness its not hard just use your imagineation