If he's logging in he must have seen your messages. My guess is that if it's important to him and he really wanted it back he wouldn't let it linger for so long. All you can do is make an effort and give him fair a chance to get it back, and it sounds like you have. Beyond that it's his lack action and you shouldn't have to store his "abandoned" stuff.
Message him, tell him you want to get his gun back to him but more importantly you just want to be rid of it. Give him a reasonable time frame, 30 days should be plenty for him to come up with shipping costs, 60 days if you're feeling kind. And if he can't make an effort to get his shit back by then you're going to sell it off end of story.