my son Jareth is also autistic. I gave him one of my own laptops when he was 4. no problems with a mouse, as it had a synaptic pad.
I set him up with a simple system - Ubuntu with KDE3 in Kiosk mode. he was locked down to a few simple tasks.
He spent most of the time browsing around the CBeeBies website or playing "BridgeBuilder-Game".
Eventually, though, I had to take the laptop off him - he got so engrossed in it that he would forgo food and the toilet in favour of getting his "fix" of the laptop.
I'll give him another laptop when he reaches six or so and is understanding that he is only allowed the machine for an hour or two at a time.
To the OP: just get a vtech or other similar machine. A real computer is just too much coolness for a 2-year-old to handle. You need to give them something that they can let go of after a few hours.
And to the "oh get over yourself" poster - grow up. This is the 21st century. In the 20th, kids grew up with lego. Now, it's computers. You don't need to be a "genius" to use a computer. Any kid can handle it if it's set up properly, so asking for hints about a laptop to give the kid does not imply that the kid is somehow gifted.