Bettas usually won't eat flakes, unless really hungry--the flakes, unlike pellets, don't even begin to resemble their natural prey of small insects and insect larvae--and generally speaking, the flakes don't have all the nutrients they need. I'd recommend just staying on the pellets, as those are usually dead-on what bettas need nutritionally. Out of curiosity, why do you want them to switch to flakes? I've always found pellets to be less polluting and easier to portion.
Also, if you like, you can shake things up by giving your betta some dried or frozen bloodworms--they usually go nuts for those--or any of the other smaller worms you can find at the LFS
. I sometimes even toss a small moth from around my house into one of my tanks--something about attacking live prey gets some of my bettas very worked up.
Whatever the case is, if you decide you still want to switch your betta to flakes, just don't feed him anything but them. After a while he'll get hungry enough to give them a try, and you never know, maybe yours will have a taste for them. Just check the nutritional facts on the flakes (compare them to pellets or other betta foods) and see how they do in that regard. Watch out for the knock-off cheap brands; those won't do your betta any good.