If he has 20k followers, he might not even bother checking messages. You should look at his posts to see if he responds to any comments, cause there are people that don't really bother responding to their followers because they don't see any point (possibly because there's just so much, or otherwise)
When you're at the point where you have tens or hundreds of thousands of followers, unless you really have a lot of time on your hands you probably won't be responding to everything you get. It just isn't feasible, especially if this is just a hobby.
As for whether it's socially acceptable; sure, follow-up, try a different way to approach him, whatever. If it doesn't show that he "viewed" it then it's possible he doesn't bother looking at random messages.