Hi, thought I would chime in with my teacher hat on…
You have been given great advice here, read and keep reading! Make sure you spend as much time as possible talking about the illustrations as well. Have your little person describe as much as they can, and retell the story using the pictures. See if he can give an alternative ending to the story.
Carry out experiemnts, grow stuff, bake things and talk about what you see happening, cause and effect etc. Make predictions and justify them. Compare your results with the predictions, how close were you? Even with the simplest of things: "If we put this cake mix in the oven, how will the heat change it? When it becomes cool will it change back to how it was before? How about heating up these corn kernels or frying this egg? Does the same principle apply to water?
The advice I would give you is this: many bright children have great memories. They can memorise books, maths facts, dates, the order of things and so on, but they don’t always understand the process. It is one thing being able to recite the order of the planets, but that isn’t important. What is important is how planets were discovered. How do they stay up there? What effect do they have on the earth? How is a planet defined? What tools do we need in order to study planets?
For example, I had a very bright child begin school in my class. He was three, and had tyhe most difficult time settling inot school. His mum was surprised, she told me “I thought he would love school, he knows all his letters, sounds and numbers, he can add and knows his shapes and colours…” She went on to say “We have watched the video of how (well known small dog) goes to school, how he doesn’t like it at first but stays a while and plays then doesn’t want to go home. He can recite the whole story to you…”
The thing is, although he knew the story, he couldn’t relate it to himself or to his experience. He is a really bright child, but is still only three and only able to deal with emotional things at a three-year old level. Take care not to expect that children can behave much older than they are just because they can think much older than they are.
Ok, teacher hat off… Me hat on…
At school I was put ahead, I was put into a class with children almost 2 years older than myself (one grade level up, but my birthday meant I was the youngest in my original grade level). They didn’t know what else to do with me. I coped just fine with all the academics, but I struggled like mad with the social side of things. I was generally quite shy anyway, but those children were a lot older than me, and as we progressed through school the difference showed even more, you try being the nearly 11 year old in a class of nearly 13 year olds. I was almost as mature as the others but lacked their experience. They talked about TV shows that were on after my bed-time, they were being allowed to play about with make-up and beginning to hit puberty…
Personal and professional opinions mixed I would say don’t go ahead with any testing for now. Teach your child all you can as long as he wants to learn, let it all start with him and and his interests. Make it fun as well as educational. Don’t push, not for a minute. As I said, the advice you have been given here is top-notch!