A: First, congratulation! Sweeping a section is always great. Good job!
Second, because it's all win without lose, there will be big bonus points for him. Most likely his rating will jump a lot. His new rating will not allow him to play in the current section.
Third, there is nothing about consolidation in chess. We always try to climb up and up, until hitting a plateau. Believe me, the plateau will come. You have no need to introduce one by yourself. We all work hard to shorten the plateau so we can move up to the next level.
Fourth, if your child continues playing in the same section, he will get used to it. It's like workout, only it's mental workout. After your child gets used the level, he will be stuck in the same level. More workout time will not help much. There are so many kids stuck in scholastic tournaments at some levels for couple of years, then they give up. They never reach the advanced level (above 1500), which they should with years of efforts.
You should give your child challenges from time to time. That's how they grow.
I will talk about playing up in another post.