Seriously how/why would he catch it?
He has always just helped them and they would leave afterwards.

Think about it this way: Ash "wins" the Alola league(because you literally CAN'T lose with the fucking pokemon that they praise in that region) and he goes onto the next region while Solgaleo sits in Professor Oaks lab.

If thats not the scenario then its gonna be fucking X and Y again except he wins but that still wouldn't make any sense.