This same thing happened to my nephew. He fell, banging his tooth of his knee. The tooth turned a grey color and he underwent the same things that you described above. After the procedures, his tooth lightened a little. Eventually his baby tooth fell out and his adult tooth came in perfectly normal.
Its great that you took your child to a pediatric dentist. A pediatric dentist is specially trained to work with children and as it is his first visit, will ensure that the visit will go as smoothly as possible. Its a great way to get your son use to seeing a dentist.
A pulpotomy is when the dentist will numb your child's tooth and remove the pulp of the tooth on the top part. Then the dentist will fill it up with a white filling to remove the discoloration and fill the hole that is made when the pulpotomy is done. A pulpotomy is sort of like a baby root canal. The discoloration is probably coming from the pulp dying from the trauma and it would be best to take care of the dead tissue in his tooth before it spreads down the tooth and into the jaw bone. If it does, then it can affect the adult teeth.
The procedure does not affect the adult teeth in any way as long as its done well.