Tracking

From your phone, no. From a junk iPhone attached to your son's Apple ID or by visiting Apple's Find My web site and logging in with your son's credentials, yes.

Your son's Apple ID can select which one of his devices will be used in Find My for others to see. When you got him his iPhone, you likely changed that registration from his iPad to his iPhone.

From a device with a different ID that your son has shared his location with, you can only see that one primary device. From another device using the same Apple ID (or from Apple's Find My web site), you can select "Devices" from the tab at the bottom of the screen to see all of the devices attached to that Apple ID.

In a pinch, you can sign in to your son's Apple ID from your own Find My page. Click on "Me" and scroll down to the bottom of that page. Click on "Help a Friend", and a sign-in screen will appear, from which you can enter your son's credentials and track all of his devices.

If your purpose is general supervision, a combination of finding him via his iPhone might be sufficient most of the time. If you need to track the iPad routinely, only my junk iPhone suggestion will give you a device that you don't have to sign in to every time you want to perform a search.

Good luck.

I believe you can, though in fact you won't be able to tell which one he is actually using at the moment unless you see it moving about.