Sickmamma (I'm so sorry I don't know your first name

), usually dogs just become more placid with age unless they develop health problems, in which case they'll become more aggressive when faced with annoyance. You probably can not teach the dog to never snap at your son when he displays these (very typical might I add) little boy behaviors, as the dog really is only warning him to not hurt it. About the only thing you CAN do is to teach your dog to have a soft mouth. As stupid as it sounds, I promise you can actually teach a dog this. Then, even in situations when the dog is biting a human out of anger and not just rough-play, the dog will learn to not use more jaw-force than you have taught it to. This is also very helpful incase the dog ever has some type of injury occur to it that you have to assess (such as a broken limb or getting it's tail stuck in a door), it will not turn and deliver hurtful bites to you in a moment of desperation. Of course if provoked further in horrible circumstances where the dog really feels it must defend it's life it might bite as aggressively as it is capable of, but in my experience I have never witnessed that scenario.
But anyway
Teaching your dog to have a soft mouth (also referred to as bite inhibition if you wish to look up more information about it online):
First you must work on decreasing the force of the bites, and then you decrease the frequency of the bites. Remember this - bite inhibition must be taught in this order. If you try to decrease the frequency of the biting first then the dog will never learn to soften his bite. In the end, you should have a dog whose "bite" will consist of placing it's teeth onto your skin with NO force behind them.
* Step one: Play with your dog and maybe get it roughed up a little (make sure this is done by you or another adult, but not your son), and allow NO painful bites. Almost all young dogs will stop immediately if you squeal (imitate a puppy yelp) in a high-pitched voice. (About 10% of puppies, however, will only get worse after the yelping. If you happen to have a dog like this, then in order for you to teach it that it is hurting you is to end the game all together and ignore the dog completely as quickly as possible so it can relate biting to you ignoring it, which is the worst punishment to a dog. If you end the game, it is very important you get away from the puppy as quickly with as little attention as possible. It's sometimes easier to have the dog tied up to something, that way you can just step back out of its reach if need-be. You could also be in a confined area with a gate and then just simply stand up and move past the boundary. Make sure you give the dog NO attention for a good 2 minutes. Remember that ANY attention you give the dog - even when it is negative - is attention. If it is really hard to get away immediately, when you begin to get up create a hand signal that means stop. You can use it anytime you want to tell the dog "Okay, fun's over. You're a jerk".) Keep in mind that this game requires you to administer NO negative reinforcement or punishments. Do not scream at the dog or shove your hand into the back of it's mouth or anything like that. All that it requires is you to yelp (or move out of the way) to get it to stop hurting you. If your dog is really excited and it's teeth are starting to hurt you a lot, than give it a chew toy and continue the game later. Too much at one time can be overwhelming for you and leave red scratches all over your hands. I know from experience that this is nooot a pretty sight
* Step two: Once you can get your dog to stop biting whenever you yelp (or stand up and stop the game) you can teach your dog soft biting and eliminate all pressure. Remember that this is a gradual process. Begin by setting a limit oh how hard the dog can bite (don't start out with something unrealistic for the dog) and anytime she bites you harder than that, yelp like a hurt puppy (or give your hand signal and leave), stop the game, and ignore for a few minutes. As your dog learns what is okay and what is not, start to gradually increase your expectations and only allow softer and softer bites. Make sure you don't do this too gradually or you will confuse and frustrate the dog and make it think that you are being unfair.
* Step three: after your dog's bites have become soft and harmless, teach it "drop it" (if she already doesn't drop things she has in his mouth on command). This is important because, one, it makes games like fetch easy to play and safer for children to participate in, and two, if your dog ever gets so mad at your child that she puts his teeth around his finger, even though she may not hurt him by her bite, she may be frustrated and hold it in her mouth, which would be really frightening for your son. Drop it isn't very hard to teach, either, so it shouldn't be too bad. Get a toy your dog likes and play fetch with it. Then when whe brings it back, offer her a treat she really loves (a slice of hot dog, a small peice of grilled chicken, a cube of cheese, etc) and say "Drop it". When she makes any effort to trade her toy for the treat, say "Good 'drop it' Laney! That's a good girl!"" I know this sounds like the dumbest thing ever, but after awhile she'll learn what you want.
* Step four: From here on out, just try and make sure your dog never touches a human with it's mouth unless invited. Make sure she doesn't get snappy and grab treats impolitely, or that she never grabs both his toy AND your hand while playing fetch. Anytime she puts his mouth on you without your invitation, then let her know you don't like it by yelping like a hurt puppy so she stops immediately. Every once in a while play mouth games with your dog. Place your hands all around her face and muzzle, and allow her to mouth your hand softly (basically gum you to death like a baby) and award her with lots of praise ("Goood dog, Laney! Good no biting!").
I know I just typed a whooole lot, but I've been up since 5 am and can't sleep. I'm soooo sorry for taking over your thread, Kay.
Good luck with your family, Sickmamma. I know that it seems like it takes a ton of time, but the reward you'll have with a dog and son you can trust together is priceless. I hope I've helped.