Max found an injured Ralts during his travels with Ash & Co. Max, not wanting to leave it to fend for itself decided to try and nurse it back to health, only to be later attacked by it's family. Ralts however became fond of Max, and defended him from it's family and Team Rocket. Max decided it would be best for it to go back to its family, but promised that he would return for Ralts once he becomes old enough to become a Pokémon Trainer.So maybe later he might come back for ralts and catch it when he is old enough to become a pokemon trainer.But it is still not yet known.