Kaepernick's kneeling, while a form of "protest", is NOT a form of free speech. He is not saying a word; he is kneeling. He has said plenty outside of these kneeling escapades of his and yes, all that talk/speech on television and so-on, IS protected under the First Amendment.
But since Kaepernick's protest is not free speech, the only other way his conduct/kneeling during a football game could be protected under the First Amendment is the clause, "the right of the people peaceably to assemble." Again, NOT under "the freedom of speech," clause of the First Amendment because Kaepernick is not speaking, he is ACTING (performing an act/action)/kneeling.
The problem with the act of his kneeling is what is suspect here; because at the particular moment he decides to kneel, was simultaneous with the playing of our National Anthem. The reason I have issues with Kaepernick's kneeling is because the reason we play our National Anthem, prior to the start of any sporting event, is to honor the proud history of our country and more importantly, the memory of the fallen who defended Her. After all, Kaepernick is not kneeling for the fallen; he is kneeling against racial disparity and police brutality. So, for me, even this noticeably quiet form of protest/kneeling, speaks VOLUMES, because in a very ‘respectful’ way, by kneeling, Kaepernick is denigrating what the moment is designed to say: "We are here to honor the memory of our soldiers who gave their lives for our country, and there is no greater gift than the giving of one's life, for another."