You are spot on as far as keeping him interested.
I did something similar with mine and I can tell you from experience.. Just waiting on parts is enough to sway their interest. I found keeping a list of things to do (like sanding and priming, parts cleaning and planning) will keep the motivation going while at a holding point.
The biggest point to remember is to have fun. Secondly, don't forget.. it's his project.
As many times as I told myself that, I kept finding myself trying to push my idea's and wants. That was tough.. lol
In the end, it was a great father son experience. I'm sure you guys are going to have a blast!