Is he a new betta? When did you get him, and how often have you done changes? And how do you do your changes? (Do you get him out?)
76-80 is quite a range. Could be too much for him, who knows. I know my bettas aren't fond of a temperature fluctuation as long as it's not too fast.
I recommend not
tapping on glass, it seems to stress out fish quite easily.
Betta fish shouldn't be bored. ..so try
1. Spend time with him. For a week, have him follow your finger
2. Make sure he's NOT sick. Check for ANY signs - I recommend this website.
3. Is there a filter? Is it too strong for him? (Unlikely, but just asking).
4. Are you sure your thermometer is working right? It's not too common, but I've seen a friend have a broken heater and thermometer. But the thermometer kept reading 77.
And also, you're 100% sure he's eating his food, right ?