I wouldn't worry about specific protein levels at all, at any age. Unless he is *extremely active*, like training IPO for hours a day, he doesn't need a high protein diet.
Eukanuba is also not a great food.
What you want in a quality kibble is a named meat meal as the first ingredient. You'll see many foods with 'chicken' or 'lamb' as the top ingredient, but you have to remember that the ingredients are weighed before cooking, therefore the chicken and lamb will lose a significant amount of water weight and drop well down on the ingredient list. Chicken meal, lamb meal, etc, are more dense and do not lose significant weight when cooked. You want to avoid anything that says by-products, or uses corn as an ingredient. I prefer to feed grain free, but many people feed grains and their dogs do just fine.
If you're okay with Diamond as a manufacturer (I'm not, but many people are), Taste of the Wild and 4Health are decent quality foods. Earthborn is a quality food (and what I feed). Fromm and Acana are also quality kibbles.