Give him a league win and then allow him to mentor another trainer in the new region. Also face different challenges because of his status.
For example had he been allowed to do well in Hoenn, in Sinnoh he could have mentored Lucas. Then at Roark's gym, Lucas could have battled the team that we battled in the game (Geodude, Onix and Cranidos) whereas Ash would face some more powerful pokemon of Roark's (Golem, a stronger Onix, Rampardos, Probopass, or something along those lines.) Ash is an experianced trainer by now. Making him clueless as to how to fight a Geodude or something like that just robs him of all credibility.
For anyone interested, by friend code is 5257-9947-2315. If anyone wants to add me, remember to PM me so I can add you too.
My friend safari contains the ground type pokemon Phanpy, Marowak and Palpitoad.
Current Y team consists of Delphox, Pidgeot, Raichu, Simipour, Charizard and Gogoat.
In training: Wartortle, Bulbasaur, Chespin, Froakie, Combusken, Pangoro.