But then he is actually suggesting that he doesn't trust his current team. I think what you said in the first line is true. He probably wants to keep forces until the battle against Paul and to get there when he's fresh, and his Sinnoh pokemon are not tired. I hope he won't use his reserves vs Paul or Barry. (although I would like to see a Snorlax vs Snorlax battle)
Well we did see what happened to him in the full battle were he trusted his pokemon at the wrong time & paid the price for it.
Believing in his pokemon is a good thing, but doing that on the wrong time is not good (which was also mentioned by Reggie during the full battle episode).
Since i am taking the example of the full battle here, i would say a few things about Paul here.
Before his battle with Brandon, he didn't believed in his pokemon that much, his battle style only revolved on power. But after his match with Brandon, he changed his style, in which he used his pokemon in such a way that each played there respective roles in the battle perfectly well. This kind of strategy doesn't work if one doesn't believe in there pokemons.
Lets say, Ash uses his Sinnoh pokemon in the match against Paul. But what are the chances that Paul would use the same line-up which he used against Ash in the Full-Battle.
I am not saying that Ash doesn't trust his pokemon. If he didn't believe in them, then Ash wouldn't have won the 8 badges in the first place.
Using new attack strategies would not help him much against a trainer like Paul, what he needs is a new way of battling even while believing in his pokemon (just because he is using his reserve doesn't mean that he doesn't trust his Sinnoh pokemon, it just means that the level of challenge he is going to face in the league is far higher than he has ever faced).
But if he doesn't change from his experience in that full battle and battles the same way, then the only one to evolve as a trainer would be only Paul and not Ash.