Welcome! It's brilliant that you're starting this thought process while your DS is happy and with a teacher who seems to get him. You're right, I think, to focus on the importance of challenging him. You've probably read some of the threads here about skipping or not - this might be something to consider in future but if I were you I'd leave well alone for now, and keep in close touch with this teacher, express your wish for your DS to have things to work at that he finds hard, ask her opinion on how to get what he needs from the school beyond this year.
There's a wide variation in when HG+ children learn to read - there's definitely a tendency for it to be early, but not in all cases. In general it's worth looking out for any sign that there might be dyslexia or a sight problem or something interfering, but as your DS is now making fast progress, it doesn't sound likely in his case. Equally I wouldn't worry about the maths facts - keep encouraging him to learn them (and you might want to make an early start on times tables - I recommend Timez Attack!) but don't hold him back from more advanced conceptual maths.
He may well just take off (in conventional academics, I think you mean?) some time soon - reading fluency is a great enabler and it sounds as though he may be catching the maths bug! - but he already sounds a great kid. Enjoy him :-)
6 is a reasonable age for IQ testing, and those scores are pretty even; no reason why these numbers shouldn't be reliable. But the kid is a better guide than the numbers.
You might want to consider applying for DYS, in case their advice is useful later. You have qualifying IQ scores and him doing his sister's maths homework makes a nice start to a portfolio, if you didn't want to do achievement testing.
