One of mine didnt eat at least 1 and a half weeks before he ate his first pellet and another ate his first after 10 minuets, every betta is different. Since he is shy, it is best to leave him somewhere where there arent a lot of people passing by and when you feed him, just put in the food and leave, dont stay to watch, and if he wont even notice it, try dunking it, the movement will make it look more interesting.
If he still wont eat after 3 days try feeding him beef, mine cant resist it (if you try this make sure there is no fat or muscle tissue as he may choke, and make small portions. Do this before a water change so if he doesnt eat it he wont be swimming in filthy water)
(also the fins are often ripped so thats no problem and they will soon heal)
(My white halfmoon spent the first 2 weeks on his heater and he didnt really calm down till at least 2 weeks after i got him)