Why do we so quickly jump to depression, when we have an unmotivated, lazy, selfish adult around? Nothing will change until dad steps up.
He is not to old to remove all the food, everything but his bed and some basic clothes, etc.
If nothing else, you and dad need to go to therapy and work out a plan, if son doesn't go, if he sees you two going, then maybe he will see you mean business.
Get some books, etc and read and make sure son sees you reading them. It may help.
Good luck. I had a step brother who was much the same way, and as much as I hate war, joining the military did him wonders.