Where can I get RCX 1.0 software? We've got a LEGO RCX 1.0 without any Software.
I've installed the USB IR Tower, what should I do now?

I've downloaded the EV3 Software from the LEGO homepage and thought I could use this, but I don't know how.

Can you tell me, how my son could play with it?
The EV3 software is not backwards compatible with RCX 1.0. 

You will need to install the software from the CD-ROM that would have come with your set. If you do decide to purchase one online, you can do so from a 3rd party marketplace such as Bricklink. 

Be sure to install the correct version of the CD-ROM (RCX 1.0) and check to see if your operating system is compatible with the software.