I've noticed periods of time where my DS8 hibernates. I've learned that his brain sometimes needs a break and a few weeks of going to the park and reading nothing but Calvin and Hobbes is rejuvenating.
I used to think that he "slowed down" but I've realized that he teaches himself things that he didn't learn at school or from me-- so I'm not always fully aware of everything he's learning. For instance, his teacher got him into an online math program and now, he seems to be doing things that he's never been explicitly taught or he's taking it to the next level without overt instruction. I've also learned that his interests are not always going to be academic but are intricate and interesting to him. Right now, he's learning Magic the Gathering which looks like a lot of fun.
My DS will have a learning explosion when something grabs his attention-- however, his interests don't last as long as they used to, so I would call them enthusiasms rather than passions/obsessions. That is kind of a nice change because DS is becoming a bit of a renaissance kid with his file cabinet of knowledge.
It's kind of a long answer- DS was like your little one. He learned letters, numbers, advanced shapes before age two and entered K reading fluently, knowing how to multiply, divide, etc. It was quite a dramatic difference compareed to most of his peers- I don't think it really slowed down. He can solve word problems at age eight that have my husband and I scratching our heads.
