Once defeated, he runs for it, breaking the Radar.
You can also do a quest that rewards the Lucky Egg...after the main game...yippee. Play through the first round of Orre Colosseum, and after you defeat Lovrina, Beluh will message you to head to Agate Village.
Some of the village Pokémon have gone mute. Senilor says he got the machine that did it from Dr. Kaminko.
What follows is a back and forth of you getting a Voice Case from Kaminko, and then headed back to Agate Village to cure the Pokémon the cry belongs to. You cure all but the Wobbuffet. As you head back to the doctor, Megg says her Shroomish has gone mute as well. Once in Pyrite Town, you heal it, and Megg tells of mute Pokémon in Gateon Port and Phenac City. Back at the doctor's house, Kaminko gives you a Cry Analyzer to fix them all. You simply talk to the Pokémon and are asked a question about said monster. Answer correctly and they're cured. The only one not cured is Wobbuffet, so Kaminko says the trainer must connect with their Pokémon. Head back to Agate, and Senilor will mimic a Wobbuffet cry, healing his pal. Inform Beluh of your progress, and you get the Lucky Egg (yay).