I don't know about the paying attention, but yes, he lacks protein in his diet. And that may very well affect his ability to pay attention.
The only protein I see listed in your post is turkey sausage and yogurt, and a small amount of peanut butter. No protein with dinner. The crackers, fruit, goldfish, jelly, pasta, applesauce, etc has a negligible amount of protein. It's not enough.
Can you make the yogurt greek? It has more protein. What about actual nuts as snacks instead of crackers? Cashews, walnuts, almonds, etc are all great snacks full of protein and good fats/oils and omegas.
If his main dinner is pizza, processed chicken nuggets and plain pasta, then it is equivalent to junk food, regardless of whether he is getting cookies chips and candy along with it.
Have you tried making him actual chicken nuggets, you know, from home. Cut up chicken, dipped in batter/bread crumbs and baked? My kids love it when I make that. It's actual chicken.
What about adding some sort of beans to his diet, they are protein rich and have fiber, too.
You could switch out the regular pasta for some of the healthier versions, a few of them actually do have a decent amount of protein in them. But you have to read the label (and take into account what a serving size is).
oh, btw, by far the best breakfast food you could give him is an egg. Scramble it, fry it, boil it, poach it, it doesn't matter. Wholesome protein. Simple, easy.