<human>: Please help me walk my (boyfriend's) dog.

I'm taking care of Boyfriend's dog for a few months while he is out of town. Dog is an energetic 2-year-old German shepard/Tibetan mastiff mix (we think). He's tall tall tall and about 80 pounds of muscle and fluff. Dog does not listen to me well. I blame this partially on the fact that I let him get away with murder the first few months I knew Boyfriend (like an aunt spoiling her niece, sort of). He also listens really well to males, but lady voices are just fairy dust that float above his head.

Boyfriend has let dog pull on the leash his entire life. I decided right away that I was going to make walking times fun for both of us and teach dog to not pull on the leash. I bought and Easy Walk harness 2 months ago when I got Dog. Every time he pulled (i.e. always) I'd stop and make him sit/look at me. When he did, I'd praise him and we'd keep going. I praise him when he walks nicely. 

He was doing pretty well until it got cold, and he got a burst of energy like a kid who's eaten the entire cookie jar.

Now, when I try to walk him, he pulls like crazy. I stop and make him sit. He does. I tell him he can go, and he immediately pulls. there is never a moment of nice walking - it's either sitting or pulling. 

We're both unhappy. I need to do something. Please help.

Can you condense the text into three sentences or less?
<bot>: Dog pulls on the leash. I've tried to fix it, but I don't know what to do. Please help us.