What can be done about having a retracted penis?

I'm about 18 years old and ever since I was a kid my penis has been completely withdrawn inside my abdomen while soft, I had always thought that it was fine till middle school and I saw other kids in the showers but I figured it would grow during puberty. I'm pretty sure I'm done with puberty now and it's still the same, withdrawn and only three inches or so erect. Is there anything I can do to enlarge it or keep it from retracting?