Just to note: if your betta is moping at the bottom, under the airstone, and started to act strange after you added the airstone, Try removing the airstone
Bettas are unique in a few ways: 1, with their labrynthe organ, they do not need airstones...in the wild, they are usually found in quiet spots, side bars, backwaters, of canals and streams...most don't like extra movement of a water filter, let alone an airstone. 2, bettas interact with environment outside of tank...it's why most of us feel so close to our bettas...they talk to us and we to them...like non-bettas people talk to their 4 footed furry friends....if your betta is acting odd, chances are he is attempting to get your attention.
All that being said, please take the time to monitor your betta for diseases: does he look like some crazy chef tried to salt or sugar him? If so, he probably has ick. Darken room and shine a flashlight on his body and head...does he look dusted in gold dust or rust? If so, he probably has velvet. Has he pooped recently? Is it one long snake of reddish brown? This is normal. Does it have white sections or is it one long whitish grey? If so, probably internal parasites.
There are some really good stickies at beginning of illness/disease section...help you rule out some things.
Good luck!