I hate to break it to you but as your son gets older the rowdiness towards your dog will get worse not better.
Your son will be able to chase him, grab him more, take away the dog toys and it is just natural for young kids to be curious about a pet.
***WEIRD, after replying first to this post, the woman I babysit for called to say her son will be late as he is 3 and thier pug bit him in the face. He had to go get stitches and be on antibiotics! This happened last night! So it won't get better if the dog has these tendancies. She said the dog never liked the little boy much and would growl but last night when he went to pick her up, she flipped around and bit him on the cheek right below his eye......that is so wild to get this call right after I posted a reply. Thankfully the dog missed the little boy's eye.
I was super worried about our older dog. We had a 13 year old dog (she has since passed) that had arthritis, she did not ever growl at either of my kids, she was patient and if they annoyed her she just went in another room. That is what a dog should do.
If he snapped, the chances are your son is going to get bit at some point unless you lock the dog up when your son is on the floor.
You don't want to have to confine your dog either as it wouldn't be fair, he would see the baby as a reason he is being punished. Some dogs just are not baby material even if they are the best dogs in the world. Pulling on your dogs ears, grabbing his paws are ways to get him used to what is to come, submissive play so it is called. You can try and do things he isn't used to, not hurting but what a child would do to see how he acts even with you doing this.
Labs are very family oriented so I am a little surprised at his behavior, typically their breed is very happy go lucky.
Call your local humane societies and even trainers, talk to them about the situation. They can gauge whether this behavior is something that can change.
You have to show your son how to interact with a pet and be gentle, get the dog comfortable but I would really be worried that the dog has already snapped and growled.
Good luck, I hope it all works out where you can keep your dog.