Although the extended family issues may not have directly affected him i know that i am constantly amazed at how much my children pick up on. And if it affects your demeanour then it affects them as well.

If he only becomes aggressive when you suggest time out, then there must be something about the time out that causes that reaction. i.e. separation issue? If he becomes aggressive at other times too, then maybe that's just his last resort.

You are probably right about the boredom issue. Perhaps you can get a few special boxes for times when you think he's about to lose it - we find that a plastic tub with water and food colouring and a few random 'special' toys very useful (they're not special, just that they get put away, so they aren't out all the time). Or get something else that he loves (feathers, dirt, trucks, lego) and keep it separate just for those times.