My son is the same way. I finally started making him drink an Athletic Greens drink every morning. He asks at least once a week if he can skip and I tell him, sure, if you start eating all of these different things. Add to it, my wife grew up a very competitive gymnast and doesn't support the eating for sports mentality. A lot of the girls she competed with had major issues with eating later in life. She'd let him eat poptarts every day if she could.
I found a protein brand called Garden of Life that my son actually really likes. He eats the bars all the time for snacks and drinks the shakes like it was chocolate milk.