SBD, as far as I understand, isn't contagious.....its caused generally by poor nutrition and over feeding.
Definitely best to avoid the pea treatment, as Gracie said.....bettas are carnivorous fish, they don't digest plant/vegetable matter very well and the pea treatment, while a few have had success, generally can just cause more digestive issues.
Frozen(NOT freeze-dried)Daphnia is a good betta-safe substitute for the pea treatment.
What sort of food are you regularly feeding your fish, how often, and how much? What is the temp of the tank? Does he appear bloated as well?