PP will not install - novice but I think we've checked everything?

Sorry to hear that you're having trouble.  Installation is usually straight forward.

How far did you get with the option to download your app from the Creative Cloud website?  And did you get as far as signing in to your Adobe account?  There is helpful information to review here: Download your Creative Cloud apps.

If you've tried that, use the chat bubble at the lower right of the support pages to start a support chat.  It's a virtual assistant that's probably going to prompt you to check the links that I just listed, but if you keep entering "agent" you should be with a real person who can help with the installation process.  The agent may ask to remotely connect your computer.  I'd allow one to two hours to get it done.  

Hopefully, you'll be up and running and back here asking how-to questions.

I don't know if you're doing the trial or subscribing to one of the plans, but the subscription is discounted for students.  If your son is middle school, high school, or college, then you should be able to subscribe to a plan a the student pricing.