Do you take your dog with you on exercise/conditioning walks/runs? The dog needs as much or more conditioning as you do. In addition to developing muscles and exercising joints, the regular exercise helps toughen his pads. I also take booties, although they never seem to stay on my dog for very long. If your dog is to carry a pack, he needs to be exercised with the pack and with gradually increasing weight over time. If you're going to hike 15-mile days, he needs already to be accustomed to that distance. Otherwise I suggest shorter days until he gets into better condition.
A thin-coated dog like a pit bull will need a warm jacket in cold weather and a waterproof jacket in rain. The warm jacket may also do for sleeping in cold weather if you have an insulation pad underneath, but I'd have something else to put over him, too.
My dog comes with me on every trip (I won't hike without him). He sleeps on a CCF pad (Gossamer Gear Nightlight torso length) and wears a jacket if it will be a cold night. He's mostly Lab, but an indoor dog so his coat isn't thick enough for freezing nights. My days are 5-8 miles (at 76 I'm not exactly the world's fastest hiker, and my dog is about to turn 10 so is hardly a spring chicken either) and we take plenty of rest stops. If it's hot weather, we start at the crack of dawn and quit by 11 am, spending the heat of the day in shade close to water. He sleeps in my tent, either at my feet (he makes a great foot warmer) or curled up alongside me. There have been really cold nights when I've wished for more dogs to curl up with!
I agree with all the above posts, but need to add that a skunk encounter would ruin both your days, maybe weeks! (Been there, done that, but fortunately not on a backpacking trip!) Vinegar helps, but it's not something we take on backpacking trips!