Recently, my son fell and hit his tooth on the cabinet counter top (about a week ago). We have notice that the tooth is discolored and is starting to turn black. What should we do? Is this long term damage or can it be reversed. He is 5 years old. The tooth is not chipped or cracked nor does it bother him with pain.

It sounds like the nerve is dying. The dentist can bond it with a white covering or even put on a crown. The permanent tooth will grow in in a year or so.

You can leave it alone as long as there is no pain, but your son probably will be subject to teasing about the black tooth. (My son was in a similar situation.)

At least have a dentist check out what's going on.

The tooth is probably dead or the pulp chamber is bruised. The tooth will probably fall out in a year or so (maybe sooner). The dentist should take an x-ray to make sure there was no damage to the adult tooth.

If there is no damage to the adult tooth and there is no sign of infection on the baby tooth there is no reason the tooth couldn't be left until it falls out on it's own. If your son starts to avoid biting with this tooth, or says it hurts when something hot or cold touches it, or if there is a pimple like bump on his gum above this tooth then the tooth should be removed.
It won't take long for the adult tooth to come in.