Hi ConcernedMummy
The things you write certainly look like many things people experience with a child on the Autistic Spectrum... as I am sure you are aware. Even if your youngest turns out to be on the Spectrum also, like his elder brother... this does not mean that he will have the same traits or tendency towards aggression. The Autistic Spectrum is as diverse as the NT-spectrum in that no two Autistics are alike.
I'm Autistic and both my children are also autistic (my daughter is the most severe and even has a one-to-one worker at Special School - a school that rather specialises in Autism). All three of us are very different from each other... especially the two children. DS (5), our youngest has some temper issues and can be aggressive, whereas DD (8) has severe anxiety issues but is seldom physical... she tends to be very passive.
So please, try not to fear that your youngest will have the same traits as your eldest, even if, like him, he turns out to be on the Spectrum. We can all be very different from each other