Yes of course there are genuine cases but you need to explore all possibilities first like for instance when you ask him to do something does he hear you?
Get down to his level and tell him what you want him to do NOT what you don't want him to do. For example 'put the toy down' 'wash your hands' 'be kind' 'be gentle'.
Praise his good behaviour, again make sure he can hear you. Like I said before if he can sit for the PS2/TV he can sit for other stuff.
Make sure there is a swift punishment for bad behaviour like sitting on the stairs until he decides to behave himself.
Don't make idle threats that you won't/can't carry out.
Reward him with treats when he is a good boy.
Does he have a bedtime routine where he starts to wind down at least an hour before bed perhaps with a bath/wash and some milk and then a story. Don't let him watch TV/PS2 until before bed otherwise he won't be able to relax enough to go off quickly.
You could try a sticker or star chart for each night he stays in bed or each hour/day he behaves.
I hope this helps, I hope it doesn't sound patronising. I went through hell with my DS from when he was about 18 months old and he didn't settle until he was in year 2.