I used to have a vcr cover that went into the opening to prevent little hands and items from breaking our vcr. I just saw online a device to protect the dvd player (I am sure if you went to Toys R US/Babies R Us you could find this or something very similiar)
KidSafe Baby Proofing and Child Safety Products, Baby Proof DVD Guard

How about gettting a locked Armoire /Tv cabinet when you are out of the room so your son can not get into the electronics as an alternative to mounting all the equiptment on the wall?
If you do go this route, make sure there are no bottom drawers which he will probably figure out he can climb to get to the electronics.

Good Luck!