re: Keeping weight on a dog

That's a pretty poor quality dog food. I suggest switching to something more calorically dense (more calories per cup of food), and where the first ingredient isn't some vague "meat and bone meal"
Better options at tractor supply include:
Victor hi-pro
Diamond High energy formula
Purina Pro Plan Sport (performance 30/20)
Purina dog chow high protein
4Health Strive (any of them)

Each of these foods have over 400 cals/cup, more fat and protein, and will help your very active dog stay in better shape.