People think sextalk/text is so hard, but to be honest it isn't, so long as you feel you can't really embarrass yourself with him.
Basically just say what you are doing to yourself, or what you'd love to do to him. Being honest is the trick I use, "if I had you here in bed beside me, I'd..."
If he or you has any known fetishes, play on that, but if you get that awful mind blank, just say "you lead the way babe
" or "and what would you love me to do then?" it's simple enough. Sure texting is a two-way thing, he's bound to give you ideas.