Is Asher Going to be Small?!

My dog's name is Asher. I got him at a rescue.

His mother was a medium/large, long hair German Shepherd mix. We have no idea what his dad is. His mom had one blue eye and Asher got that from her. Asher is 4 months and only weighs about 15 lbs. I have only had him for 4 days.

I feed him Purina, Beneful Puppy and he eats 2 cups twice a day.

His mother came to the rescue heart worm positive and pregnant. Her puppies all got sick and only 2 survived... So maybe he is just malnourished and will take a little time to get bigger.

Thoughts?