My 2 year old daughter fell on our hard wood floors right on to her two front teeth. They both turned a little grey and over time one turned more dark than the other. We took her to the dentist and he told us that they were indeed dead. Basically little kids have small roots on the teeth and if they fall and bump them they can break the root in two. Teeth need nutrients and such from your blood and indeed with the root broke there is no way to get the blood and nutrients and yes they slowly turn grey, dark grey, black and die.
In my daughters case they were a little grey and turned dark grey over a period of time but they never turned black. One was darker than the other and you could notice it. We let the tooth fall out on its own. In fact, what happens naturally is when the adult teeth come, they break the roots and the teeth die and they fall out.
It took a year for my daughter's teeth to loosen up enough (by the way they were always loose after she fell, they never got strong again) and finally they fell out.
It took forever for her new adult teeth to come back in. I would say it took 2 years for them to come in. She had no front teeth for several Christmas and School pictures. We always attribute the fall to the fact that she had no front teeth for so long, but it could have been a natural thing. I have seen kids take a very long time to get their adult teeth.
She is now 8 years old, turning 9 this summer, has two very beautiful front teeth, and you would never had noticed that she had lost her front teeth so early. It has not affected her in any visable way.
Keep an eye on the color to make sure nothing else is going on with his mouth or his other teeth, but my two cents is that you can wait until your dental insurance kicks in to take him in for a visit. Most likely they are going to tell you that they are going to leave them alone and wait for them to fall out naturally. His adult teeth should push them out of the way when they come in. I hope this helps.