Well............first of all it appears you have a rotisserie, good! I would complete the entire shell, doors, hatch, hood, and front fenders first. Second, install wiring, fuel lines, brake lines, and glass. Next, suspension and steering. Next, bumpers, lights, and fuel tank. Next, (at this point it doesn't really matter what next) engine and drive train. And of course the interior.
I also recommend that you docuement the entire process.....daily log of what you've done and things you encounter as well as lots-O-pictures. Also purchase a factory service manual.....worth their weight in gold!
But before you start, decide what exactly you want the finished car to be...as in, factory original, modified, or a track car. This way you won't be waisting time by going back and changing things. Do plenty of research, access the archives on this site and Classic Zcar Club, and ask lots of questions if you can't find answers in the archives. Oh...and stay focused on the end result. If you and your son get a bit overwhelmed by it all during the process, take a break and return to the rebuild when your enthusiasm has returned. This way you will avoid taking short cuts or doing a less than acceptable job.
Hope this helps you a bit. I only speak from my experience when I went through my 240Z. I can now look back and am thankful I did what I suggest to you. Good luck
P.S. Does your son know how lucky he is to have a dad who is willing to do such a project with his son?
Tom