At 3 it's really too early for anybody to do a reliable diagnosis on whether a child suffers from any of the Autistic spectrum disorders like ADHD or ADD or Aspergers etc. there are a whole raft of them.
Boys do suffer more than girls (I think about 80% of sufferers are boys) and it does tend to run in families but it's far from certain that your second will suffer too (If indeed your first is). If you look down a couple of threads one of the answers there has one affected and one unaffected son.
The fish oils business is a bit fashionable right now and the research is not very convincing but it certainly wouldn't hurt.
Personally I'm not convinced that ADHD and the others are anything to do with diet but it may be that some food intolerances show similar signs.
I'd put the ADHD thing to the back of my mind right now if I were you - even if it's the case it'll be a long time before he's old enough for a reliable diagnosis and there's not a great deal that can be done even then.
The available drugs help some children but many like my son find the side effects more of a problem than they're worth.
A lot of kids have a terrible two's stage that spills over into the threes and fours - if I were you I'd presume that's the case.
There are a number of good books like this one
http://www.amazon.co.uk/Toddler-Taming-Parents -Guide-Years/dp/0091875285
I presume you've explained in some way that you're pregnant and that there'll soon be another baby - this is doubtlessly worrying him - he's used to being your only child and getting a sibling is often the first big stressfull event in a childs life - they don't always deal with it well.
When my wife came home from hospital after my daughter was born, my son "blanked" my wife for about