If you're talking about the anime, he meets Richie in what i think is season 2 at the indigo plateau.
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Back in Indigo Plateau (1st season), Pikachu declined a chance to use a Thunderstone. When Gym Leader Lt. Surge was teasing Ash of being a "baby," Ash's Pikachu attacked Surge's Raichu, but Raichu knocked Pikachu out. Brock suggested that Ash use a Thunderstone that Nurse Joy offered, but Pikachu decided to grow stronger on his own without a Thunderstone, then Ash got the idea of teaching Pikachu Agility. Therefore, Pikachu may never evolve. This makes sense, since there was another Trainer who rivaled Ash in Hoenn and had a Meowth with the same story: it lost to a Persian once and then hated all Persians since then.
Well, in series #1, when he faced Brock, he had pikachu for 2 weeks. On episode 9, Ash was a trainer for 2 months. On the animated short in the third movie, Ash and pikachu celebrated a year of friendship. By now, Ash should be about 15 or 16.
Ash kill Pikachu because Pikachu is useless.
Ash's Pikachu will not evolve because Pikachu chose to stay with Ash and gain more experience and in the episode that Ash had battled Lt. Surge Nurse Joy gave Ash a Thunderstone but pikachu don't want to evolve and Brock said that pikachu has more speed than Raichu
Ash has the strongest Pikachu I do too.