What Breed Is My Dog?

I rescued this pup 3 weeks ago. The vet and I have closed down his age to 4 months in May 20, 2015. He is recently losing his back premolars, and he already has his incisors coming in. At first I thought he was a Jack Russel mix, but we met a Jack Russell that is 5 months old and my dog is much taller than he is.

What do you guys think he might be? or how big he might get?