There are a couple things that you can try to help encourage your betta to eat. I would suggest first trying to get him to eat some frozen daphnia. It's available at most local fish/pet stores and I have found that my bettas go nuts over it.
If that doesn't work, you can try soaking some of the food in pure garlic juice. Though I've never tried it myself, I have heard that many people have had good luck with this.
I've also had really good luck with feeding my bettas Omega One Betta Buffet pellets and New Life Spectrum betta formula pellets. I have found both of these at local pet stores, but it seems to vary by location.
Be sure to double check that the temperature is holding between 78-80 degrees Fahrenheit. And also check that there is not an ammonia or nitrite spike going on either.
I hope this helps, and good luck with your rescue.