See if you can cultivate some mosquito larvae. It's a very fussy betta who can resist the natural call of live, wriggly food.
Leave a bucket full of still water outside for 5-7 days. At this point, you should notice some wriggly black worm things - these are basically live bloodworms. If you can pick them up gently with tweezers, trying not to crush them, give them a little dip in garlic juice, then try and catch his attention by bringing them within his line of sight (still using the tweezers) he'll hopefully give them a go.