"Continue on. After completing the second step of the invitation, help him a few more times and arrange a few more chance encounters to invite him. Once he lowers his guard somewhat and believes you truly want to recruit him, you can begin the third step..."
"Isn't he selling Wild Pokémon? Next, he'll be targeted and hunted down. I'll chase him, severely injure all his Pokémon, including him, until he releases Larvitar. Then you'll appear and save him. I'll make him pass out, and you'll take him away..."
"At that point, don't ask for his Pokémon. Take him to the Black Ship, treat his injuries, and treat his Pokémon too. When treating the Pokémon, make sure they're all awake—at least Larvitar and one other Pokémon. Those two must be awake and witness your treatment..."
"Then stay by his side until he wakes up. Larvitar will be beside him. Invite him to join the Black Ship again, telling him that fighting is forbidden aboard the Black Ship. Other than you, no one knows he has a Larvitar, so he'll be safe on the ship..."
"Leave him some supplies and let him think it over. Let him come find you himself. Gengar will keep watch on him at all times. You can also install surveillance in his room and monitor him secretly. If he chooses to run, things will become simple..."
"I'll hunt him down again and make him regret not joining the Black Ship before he dies. Make him tell Larvitar that you'll look after Larvitar and the other Pokémon, entrusting Larvitar to you. That's the most crucial step. Once he says it, we can kill him, along with the other Pokémon..."
"Only you and I will know this secret. We'll both change identities and alter our scents before every operation.