Betta Fish: How to help constipation?

Sweet boy.
My poor guy has gotten pretty constipated recently. I’ve fasted him intermittently, and reduced his feedings. I’ve even had him fight his reflection a few times to work his muscles (and it got him to poop a few times). Aside from skipping feeding every other day, I don’t really know how to help him poop… I’d appreciate any advice as I know being backed up like this probably isn’t comfortable
.