Yes, Majestic,
Agree, try not to be drawn into confrontations and arguments except where necessary.
Agree, also to mainly walk away, but dish out some reasonable consequences where necessary, and have a few things that you don't accept (e.g. I don't accept a dirty bedroom - no or reduced pocket money if kept in that state, no arguments, but no pocket money or treats).
My son is grumpy grumpy grumpy too - I just try to ignore. I just cross my fingers and hope he's not depressed.
The rudeness - well I rarely engage with him conversationally unless he is being pleasant. If he doesn't do something he's meant to e.g. washing up on occasion, again no or reduced pocket money. I just pay for basics otherwise - food, travel, phone card SIM (£5 per month). So far, he hasn't had full pocket money the last 6 weeks.
But when he does help etc or is OK to be around I do try to be appreciative.
Also, try and live your own life (if you've any energy left), and try not to let it get you down, though it can I know make you feel awful at times.