T.C.
Hi D.,
I would be sure and take my child to the dentist as soon as possible. Since you said he bumped his tooth a while back and that the tooth turned gray, I think the tooth could now have an abscess. Be sure to have a dentist look at it carefully! They should be able to do an xray of the area and evaluate what to do next.
My son bumped his tooth when he was little. The dentist xrayed it and said it was fine but if it turned gray we should return to see him. He bumped it once more and it didn't even bleed, but it turned gray and I took him back to the dentist. They strongly encouraged me to have the tooth removed or have a pulpectomy done to the tooth so as to avoid any permanent damage to his permanent teeth. They pulled the tooth (he was 2 years old) and he just didn't have a tooth there until his permanent tooth came in when he was 6 or so. It wasn't that bad. But at least we didn't damage his permanent tooth. I would be sure to have it looked at as soon as possible since you don't want there to be any thing wrong with his permanent tooth. If your dentist says its no big deal to leave a gray tooth, you may want to consult with another professional.
Best of luck with your little guy and his swollen gum. You may also want to check if he has a low grade fever. He may be battling an infection in his gum/mouth.