is he struggling to get to the top? If so this may be a swim bladder issue. I had a simmilar issue with my betta and he did not make it. From what I have read it is normally not a fatal issue, but my betta was already old. By the size of yours, he seems young.
If it is SBD I would first recommend either shallowing the water or putting him in a cup to make it easier for him to get to the surface for air. Secondly fasting is normally required due to the fact that SBD is normally resulting from overfeeding/consipation.