My daughter fell when she was 18 months and knocked her front tooth cattywompus. It never corrected itself. It turned slightly brown, but not gray (which would have been bad, because gray means the tooth is dead).
We are having to extract it now that she is 5 years, because she has already started losing teeth. She lost both her front bottom teeth and her dentist said the top front would be next... And probably sooner than later. Some start losing teeth at age 4, some at 6 or 7.
The problem is that, if there is a tooth which had trama to it, it's common for the roots to harden and not dissolve (which is how they get a loose tooth). Instead the roots of that tooth harden and cause the permanent tooth to come in behind or infront of it. Eventually the baby tooth will get loosened but the perm tooth is crooked or displaced by it. So, we have to get that tooth extracted. The X-rays already show her perm tooth(in the gum) is shifted and slightly out of alignment. So if the 'boo-boo tooth' is taken out, we are hoping her perm tooth will be able to correct it's placement and come in straight.
Try not to brush his boo boo tooth for a while, and when you start to, only gently. Because you do want him to have that tooth for a few more years.