The best way to get him to eat it is to fast him for a day until he is good and hungry. Then he will be willing to try it. I've never seen a
betta that doesn't love peas once they try them. It took mine a while to figure out that peas were food but once they did they'd rather have peas than anything else in the world!
Make sure he sees it dropping so he will chase it. If he doesn't see it he won't find it later.
Lava loves his peas so much he uses all his strength to dive to the bottom and retrieve the ones that fell before he could grab them. Swim Bladder or not, he won't rest until he's found every piece!