I am having a similar issue with a guy, never sext before, and this has been going on for 3 weeks, I am beginning to suspect he has a girlfriend and just enjoys the random attention and rush of desirability a sext provides.
Try just ignoring him for a few days, starve him of your attention and see if he sexts or even just texts you to say hello. I myself have made a promise to not sext or text this guy again unless he initiates it with an invitation for an in person date, The whole sexting is very enjoyable, however, when theirs no actual physical follow through it can be highly frustrating, the whole building sexual tension and then having no man to vent it out on and all, which brings me to another possibillity, you could very well be providing the pre-foreplay this guy needs before having a sexual encounter with someone else-just a thought, but I could be wrong.
Just know you're not the only one that's getting yet again mind f**ked by a guy. Some unfortunatly take playing mind games to a whole new level, a year is a pretty long time to be sexting though, I hope you have had an in person date with him during the past year-if not forget the guy, he could be sexting several women for all you know, maybe that's why he is going cold-his attention is on one of the other ladies also boosting his increasing ego with a sext! x