I have 2 options:
1st - Maybe after this Shinohh League he'll go in another region, or some Shinnoh Frontier and get all of his Old Pokemon that he left ...
2nd - He goes on a quest to find all of his Pokemon and geather them all togeather (Pidgeot, Primape, Butterfree, Squirtle, Charizard ... , ... , ... , ... )
1st - Maybe after this Shinohh League he'll go in another region, or some Shinnoh Frontier and get all of his Old Pokemon that he left ...
2nd - He goes on a quest to find all of his Pokemon and geather them all togeather (Pidgeot, Primape, Butterfree, Squirtle, Charizard ... , ... , ... , ... )