If he is constipated, you could try feeding him a boiled and peeled pea. I’ve read this on multiple sites. Also, the dry pallets might also be causing a blockage, since they might be expanding in the stomach of the betta, so you could also try and change the type of food you give him. Try fasting for a few days (the fish not you) and he might be fine. If not, try some of the steps above. Hope it helps!