This could be because of it's age, or it may just be it's personality. One of my males lays around on everything. He's quite hilarious. Sometimes he seems to be sound sleep in a completely vertical position, nose down, in a plant. It's rather cute actually.
Because it's a labyrinth fish it does need to be able to rise to surface easily. It's possible the filter flow is a bit much for him and that's why he hangs at the bottom. I provide a nice wad of floating pennywort for all of my bettas in community tanks. They love to rest in it, especially at night, and can easily take oxygen from the surface w/o exerting much energy. I'd consider it a must for older guy.