You can try soaking it in white vinegar, which is often used to de-calcify shower heads (removes that white crud and opens up the little holes). I haven't any idea how long it would take for threads; I soak the shower head overnight - which is probably much longer than necessary. I've also heard that vinegar will unstick rusty bolts. Next time DH goes to put attachments on the hose, have him first wipe the threads with petroleum jelly, so that the attachment's threads don't get stuck to the hose threads.