First of all, let me say that I do not, by any means, claim to know as much as most of the people that post here. What I know I have figured out by trial and error, and many google searches 🙂. If anyone else has a better answer, please feel free.
When you say that everything that is on your iphone and ipad is now on your son's, are we talking just music and apps, or things like calendar, contacts, etc too? Make sure your son's devices aren't syncing to your icloud account (Go to settings, then icloud, to check which account the device is logging into). Since your son has more than one device, he might want his own icloud acct - which would keep his two devices synced with each other.
As far as sharing an ID, this is what we have set up. My girls' devices are each logged into their own icloud account (as described above), which keeps their devices synced together and also syncs them with their computer, and my phone, ipad and computer are logged into my 2nd apple ID (which is now my primary one). But for the purpose of buying music and apps, we share the initial ID I set up, which is no longer used for anything but buying music and apps. This allows us all to have access to anything that is purchased through that account. And by each of us having our own apple ID/icloud account, our calendars, contacts, etc don't get all mixed in together.
When you go into the itunes store from a given device, scroll to the bottom of the screen to see what account you're logged into. You can log out and log in with another account even if it isn't the account the phone is logged into. This also applies to the App store, too. To share an ID, you want each person's devices logged into their own icloud account (in settings), but in itunes and the app store, make sure EVERYONE's devices are logged into another apple ID that everyone shares. Does that make sense?
As far as apps he previously downloaded using your apple ID, he can log into the App store through your ID, then go to Updates and it should say Purchased at the top. Click on Purchased and it will bring up a list of apps that have been purchased through that account and he should be able to redownload to his new device from there. But, if he's now buying through his own ID, make sure you log him out of your account when he's done and log him into his own, if you want to do it that way.
I hope that's not too confusing and that it helped some.