I'm sorry, but the only thing we can suggest is separating them. They are not children, you can't tell him to stop terrorizing the others. They will do whatever they want. Out of curiosity, how big is your tank?
And whenever adding new tankmates (especially with bettas) you need to have a plan B. Either take the betta back to the store, take the other fish back, or out the betta in another tank or get a divider as a temporary solution. This is the ONLY way to stop it. Your betta will most likely kill the other fish if he has started attacking them.
In the mean time, the injured fish need salt treatments to heal. More info on your tank will help with the doseages needed.
Tanks- 55 gal(in progress)-Common pleco, Angels, lemon and rummynose tetra, blue rams; 29 gal planted- rasboras, corys, otos, clown plec, shrimp; 10 gal- juvenile goldfish; 6 gal planted- betta
Other Pets-Maggie(Tricolor Paint Mare), Lily(Sun Conure), Zoe and Melina(Budgies), Leonardo- Bearded Dragon, Pilot and Phoenix(Golden Retrievers), Beckham(german shepard/akita), Myrtle and Richard(hermit crabs)
A Better Betta Fish Care Video!