well, male and female bettas should really not be kept in the same tank, and if this is a community tank, it's doubtful any of the babies will survive, also the male may become sick from the stress of trying to protect his eggs/hatchlings. Or he may just end up eating them.
That aside you generally feed baby bettas baby brine shrimp, which you have to hatch yourself, so you'll need to set up a hatchery (i believe there's a pinned article for a DIY hatchery) and buy the eggs. with the short notice you'll have to buy them from the pet store which is not the best place, really. In a pinch i believe you can feed them powdered boiled egg yolk, but most breeders around here don't use that so I have no idea of its effectiveness.