Sure. If the NFL has a policy of mindless obedience, or if a certain posture is required during the anthem and for __ minutes following, he could get into trouble. 
Notice that Kaepernick didn't wave his middle finger or shout obscenities: he quietly protested. I've thought many times about staying seated during the anthem because of inequality....
I think the guy is a brave, class act. Good for him. Rosa Parks would be proud. 
Trump on the other hand......