I'd inform him that if he doesn't come to pick this stuff up by X date, it's going to be donated.

For X date pick whatever date you want the stuff gone by, if that's tomorrow, that's fine, you aren't his storage facility.

I know it's tempting to just dump it, but you'll have the high ground (you already do, but this is also for your own benefit, you'll personally feel like you gave him every opportunity) if he complains about this later, you will have given him every chance.