As someone else mentioned, it’s not necessary to do “school-at-home” when you homeschool. Keep in mind, we don’t have to always (or ever!) do a “worksheet” or use a textbook to gain knowledge. Education isn’t something that only happens between 8am and 3pm in a brick-and-mortar building, right? We don’t have to teach things (or even measure education) the same way the school does–because if that was
working for the child, then in many cases, we wouldn’t need to homeschool.
So how do you know when he’s ready to learn something new? You’ll learn to gauge that by the way he responds. Sometimes, he’ll ask questions about something, such as how something works, and it may be a simple answer, or it may lead to some research. And sometimes (perhaps more often), you’ll introduce new topics, and see if he engages–but it helps if it’s not introduced in a dry, boring, “textbook” way.
For example, maybe soon you’ll plan a garden and ask him to choose something to plant in it, and you’ll work in the garden together… and as the seeds grow, one day you’ll talk about the parts of a plant and another day, about how insects pollinate them and worms help aerate the soil, and another day, about nutrition or the food chain… and who knows what else. And he might not even realize he’s learning anything, because it’s just interesting conversation about what you’re experiencing together in the dirt and sunshine!
You might sometimes find that when you introduce a topic (say, levers and pulleys), he
runs with it and within a few days, you find yourself surrounded by “simple machine” physics models (which, uh, may or may not have happened at my house once or twice).

Or you might discover that you’ve introduced a concept that he’s not really quite ready for yet (say, multiplication). So you might back off for now, or try giving him a smaller part of the concept (like grouping items by fives) and seeing if that’s closer to his level of readiness.
For instance, if he can sort Legos into groups of five, and can tell you how many Legos are in two of those groups (it’s okay if he counts them one by one), he’s already learning the connection between addition (5 + 5) and multiplication (5 x 2)… and is that any less effective than writing it down on a worksheet? It could even be more effective, because he can actually see how to put the concept to use.
I do agree with Cat that “play,” especially imaginative play, is important at this age. Besides, it’s entirely possible that he’s learning quite a few important concepts while building with Legos. He’s probably learning things about physics, sorting, counting, patterns, etc., but just can’t explain it yet. And he’s probably developing a great imagination, as well. Try sitting and building things with him sometimes. You’ll probably learn more than you’ll realize, too (mostly about him).
In any case, you’re his mother. You know him, you know what is likely to work, and what probably won’t. You know where he needs to work a little harder, and where he excels. You know where he needs the most encouragement, too. So take what you’ve read here, and what you read in books, and just filter out what you need for him… and
ignore the rest. Really. We can all make suggestions, but we can’t tell you which ways will work best to teach your son.
You’re the expert there. You’ve already been doing it for seven years now, after all.