I feed based on how my dogs look and feel. I do not measure or weigh anything. I keep my boys very lean. If they look and feel a bit underweight I up their quantity and will feed more lamb, if they're carrying a bit much I reduce their food, and trim fat of the meats.
If your dog needs to put on weight it only makes sense to feed him more and increasing the fattier meats wouldn't hurt (if you're sensible about it, too much fat can cause issues even in skinny dogs). When trying to put weight on a dog, I will also increase the frequency of feeds. My young GSP has gone back to three meals at the moment as he was getting too lean. His breakfast and lunch are quite small, with the regular dinner as well. It' working a treat.