i would still leave a mirror attached to the tank and see how it goes over the next few weeks. he might not have "cared" at the time you tried, but maybe when feeling agressive, hed rather take it out on the reflection than himself. he might only use the mirror when the "urge" comes along. just a thought
i also never had a betta play with a ping pong ball. colored marbles are fun for them for about 30 seconds until they decide to ignore it, never to be looked at again. more plants =good, as is the rearrangement of the tank. you could try to make a little slate maze (like a cave with multiple openings) that he can swim thru. that might give him extra stimulation. or try adding a snail or two which he might like to watch or follow...or even flare at. basically, distraction/release of agression is key when dealing with biters, at least IME.
all the best