Why sinking food beats a long-finned betta
Those flowing fins are decoration, not equipment: they add drag with every stroke, so a long-finned betta cruises slowly and turns like a ship. A sinking pellet, meanwhile, falls at its own speed regardless of who is hungry, and faster tankmates or the filter current usually claim it on the way down. Bettas are surface feeders by design, with an upturned mouth and eyes set for spotting food from below, and a betta hunting on the substrate is working against his own anatomy. The fix is not more food, which only fouls the water, but food presented at his level, at his pace, in a spot he can reach without a race he cannot win.
Feeding him at the surface
Draw him to one consistent corner, because bettas learn a feeding spot quickly, and offer floating betta pellets one or two at a time, waiting for each to be taken before adding the next. A simple feeding ring floating in that corner keeps pellets from drifting across the surface and gives him a reliable target. If the filter outflow pushes food away or batters his fins, switch it off for the few minutes of the meal or baffle the flow, then restart it once he has finished. Softened pellets, thawed frozen foods and small floating foods all work at the surface; anything that sinks the moment it touches water does not, however nutritious it is for other fish.
If you must use sinking food
Sometimes the food you have sinks, and the answer is delivery rather than dropping. Soak the pellet until it softens, pick it up with feeding tweezers or draw it into a pipette, and lower it slowly to just in front of his mouth at the surface or mid-water, releasing it as he moves in. Feed him first, before any food goes in for the rest of the tank, so nothing distracts him mid-meal. Remove anything he leaves after a few minutes: a betta that has eaten his fill will ignore the rest, and softened food decays quickly on the substrate. Never simply add extra sinking food to compensate for his slowness; the tank pays for the difference in water quality.
Making the method stick for every feeder
A betta fed this way eats calmly and completely, but the method only works if everyone who feeds him follows it, because the first person to scatter sinking food to save time resets his problem in a single meal. Note the feeding corner, the ring, the filter pause and the portion on his care routine, where any feeder will see it before they open the food. Keeping that routine in KinsLe Care Teams puts the whole method on his profile, so a family member or a holiday sitter feeds him at the surface, piece by piece, exactly as you do, and his long fins stop costing him meals.