As another has noted, it's either overfeeding or dropsy. To be safe,
cut *way* back on the amount of food you're feeding him and hope for
the best. Bettas can develop a swim bladder disorder if they're fed
too much. Although it's not fatal, it can hinder their ability to
swim. If he's hanging out in an "L" shape, you're feeding him too
much!

If it's dropsy (pinecone scales), it *is* very likely fatal. There are
medicines out there that some folks have had success with, but not
always. Good luck, and hope that it's not dropsy.

dragon