For me, I would say he's perfect given his age. It's all very subjective, see?
My boy was very skinny when he was a youngster and struggled to put on weight. He was far slimmer than Aspen is, but now he's just fine. I worry less about weight and fat and more about muscle. If your dog is underweight, they won't maintain muscle, so if that's there then you know he's not too thin. I like mine to have a big slab of their shoulders and on their rumps. That will come with maturity.
Here is Shadow looking buff during the winter. See that big pack of muscle on his bum? And the ripples around his shoulders? Perfect. This photo makes me swoon

If you can't see that definition of the muscle, then your dog is either not as heavily muscled or has too much fat cover to show the definition.
He has lost 3kg since this photo but is actually a bit tubbier because it's his muscle mass that has gone due to far less exercise in the summers.
Bitches and neutered dogs tend to find it harder to build muscle than entire boys but they still can build a good solid muscle base, just generally a bit more slowly.
When people say "my dog isn't fat, he's solid muscle", you want to look at them and decide if you can see the individual muscles like you can here. If not, then the dog is fat. It doesn't mean there's no muscle underneath, but it's certainly not all
A dog that is too thin won't build muscle however much exercise they do, as you need a caloric surplus to do so. As
@Oberon says, it's the quality of the protein that is important rather than simply the percentage. You could have a food that was 60% protein, but if that protein is entirely from peas, it won't have the full range of amino acids necessary for healthy development. The majority of the protein should be derived from animal sources, or else carefully balanced from a range of plant sources to ensure that all the essential amino acids are included.