I think some fish get "depressed" for a couple of reasons. One I've seen is not enough light and the second being that I think these fish like to think they have a territory they are protecting, especially the boys. You could try getting a second betta in a second tank and put them where they can see each other.
When I had Gannicus and Castiel Gannicus would get depressed and sluggish and just hang around his log all day. At one point I was moving furniture and moved him next to Castiel and noticed that after they got over flaring at each other Gannicus was much more active and even built his first bubble nest. The boys could see the "threat" enough that they got to "guard" their territory but obviously they couldn't get to each other.
If you can't have a second betta find something he'll flare at and react to. Some things I've had work were pens tips, mirrors, pictures of bettas, videos of bettas and Cas even flared at my daughters little Big Mac MLP toy. Just give him exercise and a reason to move around for a little bit each day.