A year ago when I had a betta at work (before I knew they needed a heater and after I removed him from the vase,) I fed him Mon-Fri, but not on the weekends. He lived 2 1/2 years. He may have lived longer had he been heated.
Bettas are pefectly capable of going 2 or 3 days without food. As someone else mentioned it's actually a good thing to fast them once a week. So don't worry about leaving him for the weekend (even if it's a long one) because of the food, just be sure janitors or weekend workers aren't going to bother him.