First-if you have a filter-I would only do twice weekly 50% water changes
In severe cases of bloat/constipation I like to do Epsom salt treatment-this works as a laxative and can relive symptoms of edema/swelling in salt tolerant fish.
You need to QT him and use Epsom salt 1tsp/gal along with 100% daily water changes until symptoms resolve -but for no longer than 10 days.
Hold all food for the first 3 days of Epsom salt treatment and then offer very small amount twice a day every-other day.
I like to pre-mix my Epsom salt 1tsp/gal in a clean 1g jug (OJ or milk jug is what I use) this make dosage and water changes easier-(also add dechloranator if on city water supply to the pre-mix treatment water)
You really need to get a heat source and get the water temp up in the 76-80F area, with some Bettas (not all) need warmer water to help with digestion.