@SaltAndLight - I wouldn't worry about getting a pH tester. Whatever the pH of your tap water is, it's probably fine for him. But yes, a heater would be a good idea. 70F is below the range for bettas. This means he's cold all the time. Being cold reduces his metabolism (which increases the risk of bloating), and also stresses his immune system (which makes him more susceptible to illness).
@LTB - Excellent advice on the temperature, the food, the cycling and the ammonia. But for simple constipation or bloating, I'd use a much lower dosage of Epsom salt - probably around 0.25 teaspoons per gallon. It's only when they start having more severe symptoms, such as SBD or severe bloating, that I'd recommend a higher dosage (about 1 teaspoon per gallon). If it's something really severe, like dropsy, that's when I consider going up to 3 tsp/gal.... Also, they need time to adjust to the Epsom salt. I've seen recommendations that the increase should be no faster than 0.5 tsp/gal increase every 12 hours. (Ie: start at 0.5 tsp/gal. If needed, 12 hrs later, increase to 1 tsp/gal. etc.).
.... So, to answer your question: if the bloating doesn't go down after a day or two of fasting, then I'd start Epsom salt at 0.25 to 0.50 tsp/gal. If that doesn't work, a day later, I'd suggest using a slightly higher dosage. If his symptoms changed at any time (SBD or increased bloating despite being fasted), I would increase the Epsom salt dosage faster, and also investigate things like internal parasites or bacterial issues..... The best way that I can describe it is that there isn't always a clear-cut procedure. You have to look at the "big picture," and consider everything that's going on. There are symptoms, signs, environment (water quality, etc). Based on what happened previously and what's happening now, I decide what to do next.