What Breed Is My Dog?

When I adopted him, I was told he was a Pomeranian- Toy Fox Terrier mix. But as he is now seven months and almost 20 pounds (bigger than either of those dogs), I'm thinking that's not true. Also, I don't see any Pomeranian in him at all. He's definitely a Fox Terrier of some sort. I don't know if smooth or toy, but he's got the long, slightly curved tail of a smooth Fox Terrier. Any ideas what kind of dog I have?

Sorry the room is messy in the picture :c.