According to Jean Piaget, most children do not reach the level of cognitive development that you are talking about until they're at least 11 years old or older, and from the classes I've taught I would tend to agree with Piaget. We also know that the frontal lobe of the brain, responsible for reasoning, planning, and judgement, isn't fully "connected" so to speak to the remainder of the brain until a person is somewhere between 25-30 years old (an interesting article here). While you can certainly have your son's cognitive abilities assessed, Piaget is a pretty sound source, though I believe in light of more recent research on the brain Piaget would probably revise his final level of cognitive thinking. Be that as it may, you can read through his theory and more informally assess your son's cognitive abilities. Some kids move through the stages faster than others, though I would doubt there are many 7-year-olds who could be classified in the formal operational stage.
I don't know how old your son is, and I certainly believe it doesn't hurt to begin pushing critical thinking as much as possible at as young an age as is practical. D. Alan Bensley wrote an article about teaching critical thinking in college psychology, and he came to an interesting conclusion that professors/teachers/parents/etc. had been really going about teaching critical thinking skills all wrong because they were just so obvious to those of us who had all ready achieved CT skills. We assume that by modeling critical thinking or challenging our students/children with more difficult work or throwing them into a debate that they will learn critical thinking skills in a more or less organic manner, but really we need to be teaching critical thinking skills very explicitly. While you might not think that an article written for psychology professors could be of any use to you, he offers some good suggestions on techniques for teaching and encouraging critical thinking that I think you could modify to use with your son regardless of his age or his level of cognitive development such as:
- Motivate your students (or child) to think critically.
Arouse interest in your child in whatever it is you're trying to teach him. Tell a compelling story, for example, about when critical thinking wasn't applied appropriately. This helps your child to understand why critical thinking is important and see that failing to apply critical thinking can have unintended consequences. Additionally, encourage open-mindedness, skepticism, and fair-mindedness in your child as lack of these can be a barrier to critical thinking.
- Clearly state your critical thinking goals
I think, at your son's age, this is probably more for you than for him, but it gives you the opportunity to really think about how you want to teach a certain CT skill.
- Find opportunities to infuse CT that fit content and skill requirements for your course
This is pretty self-explanatory. If he grows accustomed to CT in everyday life then he should be able to transfer those skills over into academia.
- Use guided practice
- Provide feedback and encourage your son to reflect on it
That's just a few of his suggestions; there are a couple more but they didn't seem quite as modify-able (is that a word?) as the others.