I feel like I'm being held hostage

I'm a parent to a nearly 19 year old ASD young man. He's dropped out of education, won't engage with life, job searches etc. I suspect he's depressed and have spoken to the GP about this.  The GP can do nothing unless my son seeks help for himself.  He won't engage with friends, never leaves the house,  promises to apply for jobs/apprenticeships etc but lacks the initiative to do so. His behaviour towards us, his parents has got worse and worse. He speaks to us like we're something he'd wipe off the bottom of his shoe, or walks away when we're trying to speak to him. He knows he is loved unconditionally and we're not going to throw him out, so he can get away with anything,  right? How can we encourage him to get help/engage/be nice?