Actually it'll be possible once in 20 attempts, since ANY save succeeds on a natural 20.
I'd leave it as the feat.
Just realise that if the javelin ends up sticking out of you because it HIT, you HAVE caught the javelin.![]()
Or maybe make it so it costs an Action Point to attempt, if you're using the system. It lets him try it a few times a level but that's it.