I totally agree with what is already said here, it is waaaay better to have a dog a little on the light side rather than on the fat side, especially our big and giant dogs. Mind you all dogs are different and have a different metabolism. If both you and hubby think you might want a tad bit more weith on him, I would try to feed him 2 cups 3 times a day, that will only be about 1/2 a cup more than what youre already feed, but even if it is just a slight upting in amount, if you feed more times a day it is easier for them to digest and really take up the nutrition in each meal.
But I think he looks really nice too, so I wouldnt worry at all.
Good Luck and happy eating
Marit