Agree, an astute teacher would see from that answer that your son is well ahead in understanding the mathematical concepts. Sadly you will be lucky if your son has that teacher, at primary level (? any level) they are not common. My experience so far is that practical arithmetic is not being taken far enough soon enough in favour of activities designed to develop conceptual understanding. I don't have anything against the latter - but as soon as a child "gets it" they need to be shown the next step, eg how to add 2 and 3 digit numbers in columns, and not held back.
My daughter knew her times tables properly at the end of year 2, as did most of her maths "set". She had a good conceptual understanding of division. Year 3 - more times tables. Year 4 - more times tables, and division but not beyond the tables and not usually with remainders. Eventually about half the top set were shown properly how to multiply but only briefly and mainly because the teacher suddenly realised many parents had done it in the holidays in desperation. By this point my daughter was getting muddled about division, mainly because she tried to make it more complicated than it should be (can't be easy, or why would it have been left so long to do it at school?).
Your son reminds me of myself at that age, mathematically (though I don't think my parents told me about i :) ). The one thing I am glad I got at primary school was loads of practice at arithmetic. Your son may well not get that at school, so I would encourage you to give him more at home. Fluency with numbers is the greatest gift you can give him to support his mathematical development when he is older. And nothing wrong with moving on to algebra. Talk to the school as soon as you can and see what they are able or willing to do to stop him becoming bored and a menace :) And enjoy!!
You might like to look at "mathletics" online (schools can subscribe to it, so something to ask about or suggest, or else do at home). There are also various sources of problems to solve. I hope your son can fly with his ability :)