Too skinny?

Rio is a tall and lean pup. I can feel his ribs and his spine, he doesn't seem like he has much body fat. He eats well and is fed over the recommended amount of his food along with numerous treats and stuffed kongs throughout the day. He just doesn't seem to add any body fat. He's a VERY active dog and I'm beginning to suspect he may have a bit of sighthound in him. He has a very lean build as well.

My vet has had hands on him recently and believes he's fine, a little underweight but not significantly. My vet prefers keeping him on the lean side to prevent future joint problems and other obesity issues. He also says, and I agree with him, that it's easier to put weight on a dog than get it off. He also said that because he's still a puppy and still growing, he's hesitant to recommend we bulk him up too much. He's had a recent fecal and blood work done, nothing there. Our agility instructor thinks he's perfect as well. He has lots of energy, healthy coat and skin.

That being said, EVERYONE else we encounter keeps telling me he is too thin and I'm really starting to second guess myself and his weight. We took him to the groomer yesterday to get a bath and she really cut into us about how thin he is and said we need to feed him more. The lady at the pet store said the same thing and some people at the dog park today kept going on about how thin he was. I've had someone threaten to call AC on us because they didn't believe we were feeding him enough.

I'm thinking I should add some weight to him just to avoid the comments, but I don't really want him to get overweight. I have a hard time judging body condition because I'm so used to overweight/obese dogs/animals and the descriptions I read online don't seem detailed enough for me to understand.

I know that it's hard without getting hands on him, but does anyone have any suggestions on how to judge if he is really severely underweight? Any ideas on how to safely add some body fat to him without causing him to get too fat or filling him out too fast?

I just hate people assuming I must be starving him.