What could be causing my son's tooth to hurt so badly?!

This doesn't sound normal and quite weird that nothing showed up on his xrays, it almost sounds like a nervy pain if he is jumpy and sensitive when he bites down, decay would be the obvious conclusion but it could be a dead or dying nerve in the tooth, you didn't say if it was an adult tooth or baby tooth, if its a baby tooth get it extracted for him, ne need to keep it at all, however if its an adult tooth then get back to your dentist, ask to see another dentist and ask for them to check the tooth for signs of life using Ethyl Chloride, this is a ice cold liquid that they put on a cotton wool roll and test the tooth/teeth for feeling, this hopefully wil determine if the tooth is dead or alive and also it may not be linked to one individual teeth as at any age it is often difficult to point to one tooth when in pain, sometimes an upper tooth pain feels sore on the bottom jaw? Good Luck

I am a Dental Nurse in the UK in Oral Surgery

He could be feeling the pressure from his canine forming/trying to erupt. Usually these teeth come in around the age 12 or 13. I remember when my wisdom teeth were forming I could feel pressure and I was in pain for a couple of days. If he is having sensitivity pain, try using sensodyne toothpaste (for children) to make sure his tooth is not just sensitive. I really hope this helps. I hate when children (or anyone) has a tooth that bothers them. :(

Me, RDA :)

it may be sensitivity in that tooth