Hello Jillie, welcome to our forum here...
I'm Sorry to hear about your son's betta...all you can do is keep treating him and hope for the best. Keep the water good and clean and up around 80F if possible. I do hope he makes it ok for you.
If he doesn't and you choose to purchase another betta...look for the ones who are in clean water and who look flared and fiesty. It's a good sign if they have a good bubble nest going in thier cup/bowl too. Check the fins out very carefully...look for tattered ragged fins and pass those fish up. Find out from the pet/fish store when they get thier betta shippments in..and be there on that day...you have the best chance of getting a healthy betta that way.
There is one store which this is an exeption....Walmart....if you are buying your fish from walmart..it's probably not a good idea. They are notorious for offering poor quality bettas..at least every walmart I have ever been in. I think those poor fish sit in a warehouse somewhere until they get shipped to whatever store they are assigned to and who knows how long that takes!!?? I have been in that store a few times on the day they recieved thier betta..and they looked terrible!!
But...I do hope that isn't necessary and that your little guy pulls through just fine..it is possible, keep trying! <img src="/images/graemlins/smile.gif" alt="" />