Australian Shepherd Breeding Size

I have a male Aussie that is now 7 months old. The breeder I got him from said his mother was 20 inches at the withers and his father was 23 inches at the withers. She knew I really wanted an Aussie that would get up to standard size especially now that the Mini and Toy craze is going on. I wanted to possibly show him or use him for breeding in the future so it was important that he reach at least 20 inches. I think she may have lied to me just to sell the pup. He is 7 months and his shoulder height is not even past my knee. His shoulders are "about" knee height. I had seen photos of his parents, but sadly (at my own fault) I never saw them in person so I just took her word for it.

Do you think he would be ok to breed even if he only got to the 18 inch mark?
I'm not sure what to do here. I doubt he is going to grow anymore being that he is already 7 months old.

I have attached a picture of him at 5 months old.