I am going to take a guess and say that the tooth is dying. And that there isn't much you can do but wait until it falls out and is replaced by a nice clean adult tooth. I suppose you could get a veneer put on it, but that seems kind of silly, and I can't see any reason to put a 2 year old in the dentist chair for all that when it won't be a happy thing for him to do -- not due to pain, but because he won't want people poking in his mouth, nor will he want to sit still all that long.
As long as it doesn't hurt, I'd leave it alone, knowing that it would still have turned color, even if you had brushed it religiously the whole time . . . if it abcesses, there will be a big bubble again on the gums, and then it would need to be pulled. Prior to that, you'll just have to live with it -- and if you're lucky, it'll fall out on its own rather than having to be pulled.
My 4 yr old had a front tooth that turned grey about a year after a fall, and it abcessed. The dentist who pulled it put novacaine in that sprayed back out thru the abcess -- it never did completely go numb and he traumatized her when he pulled it out. She looked as if she'd just walked out of a war zone when she and Dad came back to the waiting room, it was really sad. Later, when she had a few teeth pulled to have braces, we talked with the specialist who was going to do the work, and he told us that it is very hard to get baby teeth to be fully numbed up. (That dental specialist is awesome, having done work for every one of us except the youngest now, and I definately trust his answers. He also did a great job with our daughter when she had to have teeth pulled as a young teen after having that bad memory of having the baby tooth pulled)
Good luck. I would just grit my teeth and put up with the discolored tooth as much as you hate it. (I know, I hated our little gray tooth, too, but it's not fun to have them pulled out, either)