I think its probably a combination of his dyslexia, covid and possibly tbf, going through a lot in his family life. Its not at all uncommon for students to take an extra year and as mentioned, Princess Beatrice did it, and that was without Covid.
Yeah, it makes me smile how often Tessy makes out like she is sitting up each night helping her kids with their homework when actually they have been at boarding school (which I don't have an issue with). TBH for the boys boarding school has proved its worth, giving them a stable life, diluting the crazy life of their mother.