There's not a whole lot the Doctor can do. Soak it in very cold water several times a day to help with the pain and swelling. After soaking and drying it off, apply a drop of peroxide to kill any bacteria that may be present. If it bubbles, it is killing bacteria. Keep it clean and dry, but don't apply bandaids as they allow for moisture to build up. Instead, wrap with plain gauze and tie it onto the finger, or use that elastic tape that sticks to itself to keep it on the finger. Don't cover the area with tape where the nail plate injury is. It is black and blue due to bruising and/or blood that is trapped under the cuticle and the damaged nail plate. More than likely, hte nail will fall off as it grows forward. Due to the injury, hte matrix (where the nail plate cells are born) will not produce nail plate cells for a few days or a few weeks, so this will take some time. As new nail cells begin to develop, it will push the damaged portion forward, revealing a dent or a raised bump where the damage occurred to the matrix. If he plays ball before the nail plate has completely replaced itself, you may want him to wear a splint on the finger to prevent further damage. It can take anywhere form 3-6 months for it to heal and begin to grow normally again.