We must remember here, biting is most often a
learned behaviour. This doesn't mean you have purposly taught the dog to bite, but you may have unintentionally.
For example, you may want to take a toy away from him, he growls, you back off. You have just taught your dog that growling pays off. This is the same with biting and most aggressive behaviours (even fear induced ones). We are all capable of doing this. When i was training my kelpie puppy i unintentionally reinforced a fearful reaction to umbrellas. I opened the umbrella, she went balistic barking and whimpering, i quickly closed the umbrella and put it away in worry. She learned, barking drives umbrellas away.
These behaviours are very hard to curb. Unlike barking, you cannot ignore a dog if it is mauling you!
I would definetly recommend finding a good trainer in the area, plus a whole load of desensitization to teach your dog that visitors are great, kids running past the fence are friendly and have treats, and grandma is never to be growled at!
It also sounds like your dog has lousy bite inhibition. This is probably not your fault, dogs learn alot of this when they are with their litter mates. What age did you get him at?
Do you playfight with your dog? This isn't too bad if you do, in fact it can help strenghten bite inhibition if you do it correctly. If not, don't start now. Although it can help teach a "soft mouth" it should never be done unless your dog reliably downs on command even when highly excited.
If you do playfight, have you been accidentally reinforcing biting?
And, last of all, have you ever really hit your dog? p.s not accusing you
I've seen so many cases where dogs learn aggression from their owners, so i always ask this before i recommend anything.