As far as we currently know, you can level up twice after breaking through to open heaven. If he breaks through to fifth rank, his max will be seven. With the world fruit, you can raise the level by one, so it will be the eighth. Yang Kai being Yang Kai and always fighting above his class, at rank eigth he can compete with rank nine ultimate powerhouses. Probably all of his current enemies will be crushed under his heel. A pity he doesn't want to wait for the sixth rank material because then he could "easily" get up to the ninth rank. But he has always been an impatient fellow who doesn't like to rely on anyone. Proprietress Lan will be totally out of her mind if she witnesses Yang Kai as a fifth rank loser after everything he and she went through to gather high rank materials.
It does show how shortsighted Yang Kai is, though, that he didn't secury a sixth rank material for himself just in case already ages ago. They don't have those in every grocery store, but he should have been able to secure one by a genous exchange. It's still only mid range material, after all. He knew the cave heavens and paradises will want to either kill him or make him break through at a lower level, so if he had had a sixth rank material with him just in case, he could now use it. But Yang Kai is a person who never has had a Plan B. This is totally in character.