By saggy belly do you mean the horse has no back muscles/topline to tuck his belly and work, or do you mean a hay belly? Or perhaps even a bloated belly? Saggy could mean a lot of things.
My mare's belly became 'saggy' when I worked herafter she was out of work for a few months to have a foal. I first thought it was worms, then bad hay...but that wasn't it. She had simply lost the muscle that she usually used to keep it 'tucked' and neat. Once I got her working well again and using herself correctly, it dissapeared in a matter of weeks.