Or, in other words, is he just ignoring me? 
Obviously not expecting people to diagnose on the Internet and I will get him checked but was curious about others' experience.
DS is 8 and is going through a phase of being quite grumpy - when he's good he's lovely and when he's not, he's really not!! He ignores instructions a lot and DH and I get cross about having to keep repeating ourselves. Anyway, this morning, as I turned the tv down for the hundredth time I had a bit of a lightbulb moment and wondered if he has problems hearing. There's some history of hearing problems in both our families and I had glue ear as a child.
Mentioned it to his teacher and she hasn't noticed anything but I just wondered, could it be possible that he's concentrating really hard at school etc so that no one notices a problem and then is tired and grumpy at home and 'switches off'? I know when I had hearing problems no one noticed at all until a routine test showed I had almost no hearing at the top end of the range.
Thanks if you've stuck with me this far.