Barry,
You need 1 Hopper and 1 Joey if you can live with a total of 3 tuners ($99 installed). Otherwise you need 2 Hoppers (possibly $300 installed, maybe less depending on your status). All your existing wiring can be re-used. Whether to get a stand-alone for your son is up to you, but with the Hopper/Joey system, your choices are more Hoppers or Joeys, nothing else. In my house with a family of 4, I have a single Hopper, but most of our viewing is on the main TV. If you attempt to get 2 Hoppers, go through a local installer, not the Dish website or DIRT (that won't work, as you can see from a few earlier posts).
I'm guessing you pay $10/month for the 508. A Joey is only $7. A Hopper is $4 more than a 722. A second Hopper in leiu of a Joey is also only $7. So your net monthly cost increases $1 in either a 1H/1J or 2H config. However, Hopper uses about $1 less in electricity per month than a 722, so there is really no increase at all. Joey uses even less, so you'll probably actually save money.
Keep your sling adapter and use it with your Hopper. You can transfer all the recordings you want to keep onto your EHD which you can then plug into your Hopper and watch.
There is a first look thread somewhere in the forum that gives you a lot of detailed info about H/J. Coming from a 722 and 211, I can tell you the upgrade is well worth it.