Toddlers and dogs

We have an amazing rescue 4 year old lab mix. He came from a very abusive home, and we worked SO hard the last two years. We’ve gotten to the point where he’s able to be left alone without destroying the house, you can take any food/treat out of his mouth without growling, and he knows all basic commands. When we had our son his temperament changed a bit, as expected. He was used to having all the attention and now it was cut back pretty significantly. They have always had a great relationship up until recently. When our son started crawling, he showed signs of being “fed up” with the constant moving, the same with walking. Now that our son is full blown running, his temperament has gotten worse. I’m trying so hard to teach our son gentle, and pet and easy, when he approaches him, but he doesn’t really get it and just hits him. (He’s almost 13 months) We get the very occasional pet and a hug from him haha. Our dog has never snapped at our son, but he started growling at him in the last few days. This obviously makes me extremely uneasy. I separate them when I see the distress in our dog. But I cant keep him locked up forever. Unfortunately until we move In April we won’t have a fenced In yard. Rehoming isn’t an option-we love him too much! Anyone have any tips or suggestions on how to nip this in the butt before it gets to the point of him snapping?