Literally just use it. I had the same problem for years and years. Solved itself with a little conscious effort to use it more. If it's more sore than the rest of your fingers that's good. Now it's actually a power player on my fret hand. Work on getting a clean pinky to _____ trill with all 3 fingers you're used to using. Don't like focus on that, but every once in a while after you've started using it more check back in and be impressed by the progress you didnt even realize you were making.