Is my Malinois scrawny? General Chat

My 18 month old male Belgian "Miles" weighs 54 lbs. He is long and thin to look at from above and you can see his ribs very clearly. He eats about 4 cups food a day but he is still very slim. Miles was very skinny with hip bones visible when we rescued him at 10 wks. My husband says Miles is still a growing puppy and he'll bulk up but I'm worried he's always going to be scrawny. How long does it take for Malinois to fill out?!

What brand of food are you feeding him? Not all food is created equal... Have you talked with your vet about this? Have you had him tested for parasites?

Miles just had an excellent review at the vet and no parasites. My husband took him in and Mile's weight was not commented on. We feed Miles and Jade a mix of the yellow bag (beef & sweet potato) and the blue bag of puppy food (lamb & rice) from Costco.
He eats enough, and poops regularly but he is very active and must burn it off.

If you can afford to feed them a better food it would probably benefit them. I don’t know about where you are, but the Costco dog food where I live is made by the same company that makes Diamond dog food and Taste Of The Wild, with the Costco food being the lowest quality, the Diamond a little better and the Taste Of The Wild the best of the three.