Smile and nod.
Don't offer them any information on your son. My favourite line is 'Good for them' (to be said in a positive manner) then walk away, still smiling.
I often find they get bored with your lack of a reaction. Try to take it all with a huge bucket of salt. I totally agree with an earlier post, usually when someone boasts it seems to come from some sort of insecurity.
Good luck with your son. Better for him to take his time and work through the books thoroughly and proficiently, than rush through them too quickly in which case the exercises become ineffective. You have plenty of time. You don't want him to peak too early!