Perhaps you could write his folks a note and let them know that you will be putting all of his things on the porch (lawn, sidewalk, dumpster) on a certain date. You can include a short note of goodbye if that's going to give you closure. And on that day, put everything outside for them.
If they come and get it, fine. If they don't, toss it.
Either way, rehab rules, regs, and recommendations are not brainwashing. Sometimes our addicts are more honest with them than they've been with us -- they could have his best interests at heart.
And YOUR best interests lie in getting on with your life. Or at least that's how it looks from here...
Good luck!!