I had a dog like this, and I got what looked like a double-ended leash and attached him to my other dog, who was well trained, and it worked. He was an aussie, so he wasn't being dragged by my pug, but he wanted to go and didn't seem to even notice the leash, whereas when I had them on a leash splitter, he was still planting his feet. If you can find one it might be worth giving it a try? Before that I tried encouraging with treats and putting him in a harness to walk, and the treats worked for a while, but it wasn't a good idea to make him treat dependent so I stopped.
