First, if you phone Sky and tell them you lost your job and are leaving they MAY offer you deal to pay less each month, it has been known (and they dont like losing customers).
Your only three "free" options are Freeview (via an aerial), Freesat (via a dish like a Sky dish), or to keep your Sky box but pay no money to Sky and just watch their "free" channels. They call this Freesat from Sky and you get 6 HD channels. I think you lose the "record" facility on your Sky box though. More here:
http://www.sky.com/shop/freesat/home/
Both freeview and freesat have some HD channels though not many, so you need to buy a box capable of watching HD.
There are boxes, normally called PVR (Personal Video Recoder) that include a hard disk and allow you to record programs like you can with a Sky box.
Probably best if you have two boxes so you and your son can watch and record what you like. He will of couse need a link to the Sky dish if he wants freesat.
Humax make good PVRs.