I can rememer clearly a friend saying to me 'I used to think I was a good mother until I had XXXXXX and then I realised that YYYYYY was just a good child'. This really stuck with me... and my youngest (of 3) was far, far more difficult as a baby and pre-schooler than my other 2. Thankfully she is now 9 and great... I am aware she might become very difficult in the teenage years. I think it is because she is different child to my other 2. She is very wilful and has always been determined to make her presence felt.
What is your son like at school? If his behaviour is poor at school too, maybe there is an underlying cause for the behaviour? Discuss this with the teacher if this is the case.
If he is only like this at home... then this is harder...
As far as the omega 3 goes.... good you get in omega 3 eggs, bread etc? Fish etc. My son takes omega 3 intermittently and at the moment I'm trying to do this with his diet. He has eggy bread etc for breakfast... you can get omega 3 eggs, bread, margarine? etc and it is in fish naturally.
Good luck with your son... sorry I'm not much help here.... He probably has no idea why he's behaving like this, but can you ask school/GP etc for help now to prevent further issues in the future. They SHOULD listen to you... you are an experienced parent who just needs a little support here......