Aw, poor boy's been through a lot—and so terrible that he'd been treated so poorly by his previous owners! But it's good that the mange has cleared (meaning his immune system is repairing itself) and that he's gaining weight back. He looks like a healthy young field-type lab with loads of energy. It's good that you're up for that (and so sad when people get a lab not understanding what they are getting).
Welcome, and best of luck to you both!