Nothing will keep a dog off a 900 fill down sleeping bag; dogs know what's comfortable! Just keep his claws trimmed short and don't leave him alone in the tent.
Assuming the dog, like my late pooch, curls up in a ball at night, the Gossamer Gear Torso Length Nightlite pad is just perfect (mine was a 75-lb. Lab-Golden cross). Because my dog would periodically wake up, shift position and then turn around three times before lying down (I called him my Robert Benchley dog), I couldn't keep a cover on him, so I just used a lightweight doggie coat on him when it was down to freezing or below. For the Weimeraner, you might want to sew a nice fleece coat or cut down a child's fleece jacket to fit.
I actually carried the Nightlite pad, since it really helped pad the back of my pack. My dog carried a piece of equivalent weight. I don't know about carrying three Nightlite pads, though–might be overkill.
I've spent a few cold nights in which I'd loved to have two more dogs! They do add considerably to sleeping warmth–but unfortunately also to the condensation.