Hm.
First, I think he looks all lab to me. He might be a mix—he is tall, but within breed standard for a male—but to me has the look of a field lab.
However, he does seem a bit thin to me, and at that height, 50 lbs is indeed pretty slim (low end for males is about 65 lbs). You should be able to feel the ribs but not see them—and if you are seeing both ribs (which I see in your photo) and the pelvis, then I think he could probably stand to gain a bit.
I don't think my dog is a good comparison, but: I have a female English lab puppy (4.5 months) from a show line that is bred for very full substance. Her father is probably in the range of 90-something lbs, and her mother up around 90 (though the breeder does think that the mother is a bit overweight). I expect my dog's final weight to be somewhere in the 80s. But English labs have very different body types than field labs.
Our pup is on Purina ProPlan Sport 26/16. She came home on ProPlan Focus large breed puppy food, but we switched her to her current kibble, on breeder's recommendation, at about 10.5 weeks. She is not on puppy food. I add fresh fruit / veg (blueberries, apple, carrot, sweet potato—a T or so of a couple of these) and a dollop of plain yoghurt in the morning, and a T or two of chicken boiled w/ rice and veggies at night. She is fed 3 cups total (ie. including the fresh fruit, etc.), 2x a day (that is, about 6 cups per day). At this rate of intake, she is gaining about 2 lbs a week (it had been 3 lbs a week, but she recently slowed down). She is eating at exactly the level the breeder's dogs eat, but she is also eating more than most lab puppies. We are following the breeder's instructions on this, and I monitor her substance daily, as when she gets older, we'll reduce her intake so she doesn't become overweight. At about 4.5 months she is around 18 - 19" at the withers (very hard to measure her, as she's so wiggly) and probably about your dog's weight.
But. That's for my dog and her line. Your dog seems to be eating a total of 4 cups a day (right?), but is still hungry. And is a bit thin. Obviously you don't want an overweight dog—but with his body type and activity level that doesn't sound like an immediate danger. Is he on a puppy food for large breed dogs?
If he were my dog, I would probably increase his intake a bit (or add some fresh vegetables, a little boiled chicken) and he how he responds.
At his weight, I would not reduce his food to 3 cups. That would be fine, I think, if he were at his mature weight, but he does seem slim to me—in part due to his past abuse—and I would not reduce food now. Others here may well have other advice.