Kids at that age are likely to be copying what they see adults do,if the child is left under the care of a house-help most of the time find out if she pinches the kid,or if he goes to school,he must be getting that from classmates/schoolmates.
Talk to your son,explain that it is wrong and painful to pinch others,if you have to punish bad behavior,avoid physical punishment like pinching,smacking etc.
Instead set rules,if possible write them down somewhere,and spell them out to the kids. the day they mess,they know it is wrong and know what to expect.maybe they will miss out on an outing,or start a rewarding program,for every good thing they do they get rewarded,every time they mess their rewards are reduced.
Avoid shouting at kids,they get confused,also watch your language,the way you address them is the way they will address you,they will also address other people just the way you address them!
Monitor what they watch on T V,and spend more time with them,then you might find out where the anger is coming from.
All the best.
The real measure of your wealth is how much you'd be worth if you lost all your money. ~Author Unknown