I would recommend using Kik and Skype. I believe there's also toys that you can try and get, but for long distance, which is like a blue tooth connected dildo and flashlight that lets you both feel each other's genitals long distance. I don't remember the toy much.
Also, depending on why he's away, you can try and make a list of secret words that he can choose from to text you so that are tasks you have to do and sext him later. For example, if he says you should take out the groceries from the trunk of the car, it means you need to anally penetrate yourself for him with toys and send him pics.
I am new to this completely, so these are totally random ideas I have, but hope it helps