My advice? Get a big, black contractor trash bag, load the boxes of his stuff into the bag, and dump it in a dumpster somewhere. It's been over six months. If it was remotely important to him he would've found a way to contact you for it. Pitch it.
Well, We'd be doing that except for 1 very large bin that's packed with comic books...I want to keep them and see what's in it, but my roommate says we legally cannot until we contact him and give him notice and time to come get it. But we don't know where he is now or have any way of contacting him.
What I've found online is that we have to go through the court system, and at the end of that the city would most likely seize any abandoned property, auction it, and the sales would go into the city's general fund.