He's probably upset by the Platies and the Neon Tetras. Neither are really ideal tankmates for a Betta. The Platies because of their bright colors, and, in the case of mine, their tendency to pick at the Betta mercilessly. The Neon Tetras because they are notorious fin nippers, and the Bettas can't seem to get out of their way in time.
Also, if there's a current, then that's probably upsetting him.
If possible, relocate him to a different tank. Most Neon Tetras will eat his fins up pretty bad, and the Platies probably have him pretty stressed out.
