Our Dalmatian has bit a number of people including our son. I have filed his teeth down to prevent damage, what can we do?

Hello-

I have a five year old Dalmatian very alpha male who has bitten people, including my kids in circumstances when he is frightened or is "guarding" me. He is not a bad dog or a mean dog. He may not even realize that it is my son when he bites. It seems to be an automatic response. For example, he was lying in a curled up position, next to my chair, the wall and the desk, no room to move, and asleep. My son came up to him and reached down to get his collar to take him upstairs. The dog bit his hand, in a knee jerk reaction without determining who it was he was biting. I've had his teeth filed down so that a bite is less damaging, but I cannot expect my nine year old to always remember how to approach the dog. Normally, the dog sleeps in my son's room, and stays by his side when he is not with me. Is it possible to teach the dog to stop biting? We got him when he was a year and a half old from a guy who kept him for eight months after finding him running down the street. No one claimed him or placed any ads looking for him. He was neutered and trained to sit and stay out of the kitchen. Please let me know if there is any way to change this biting behavior. Otherwise I'll have to have him killed, as upsetting as that would be.

Thank you,
Doree