Gunner is gorgeous! A DSLR is excellent to use because you can use a higher shutter speed to stop the action. For lower light, I'd suggest you buy a single focal length lens that has a lower f stop which allows more light to come in. I started with Canon many years ago, but for the past 25 years I have used Nikon. A 35mm prime lens would be a good lens to buy. For anyone interested in photography, a book I'd recommend to get a much better understanding about how aperture, shutter speed, and ISO work together is Understanding Exposure by Bryan Peterson. All of his books are excellent, but this one is great for people of all levels. There isn't a lot of reading, and the photos are so wonderful to illustrate what he is trying to explain. I have a few of his books, and this is the one I'd recommend to start with. I hear his online courses are also excellent. One day when I have more time, I'd like to take some.
I use one of Katie's balls to get her attention sometimes in photos, but food or a squeaky toy work great, too. Just keep practicing, and give Gunner time to get used to the camera. Even though I use digital now, I still take photos like I did with film, although with a fast moving dog, it's wonderful to keep taking photos until you get the expression you want. I look forward to seeing more photos of your beautiful little boy.
Understanding Exposure, 3rd Edition: How to Shoot...
This looks like such a fun class:
Pet Photography Online Photography and Photoshop Courses at The Perfect Picture School of Photography