He became one with his other half. Meaning he joined with Sora once and for all, and now Sora and Roxas no longer exist as two seperated beings. But as a whole like it was ment to be. Now body and soul, and heart is fused together to form one complete being. Sora is complete now. End of story. Roxas is now Sora in a way, and Sora is Roxas in a way. But Roxas is fused within Sora so Roxas can be considered no more in a way.
Me personaly belive that Roxas will not be mentioned. Meaning he will not make an appearence in the next Kingdom Hearts because he is with Sora.