We've been dog owners my entire life. The first thing to do is get him checked at a vet and rule out anything serious.

Was he sleeping and your son surprised him, or was he resource guarding? If he's had a rough early life, responding that way to either isn't surprising. One time I tripped and accidently kicked my dog who was sleeping, and she bit me. She felt terrible about it afterwards, based on her reaction. I didn't react at all since it was my fault. We got her as a puppy so she had a very good life and she still reacted that way.

It sounds like you have a good dog, so he should get training. He needs it in general, but especially for resource guarding. This stuff can be worked out if you're willing to put in the time.