He is the rightful HEIR.
Nothing can change that.
Game of Thrones Season 1, Episode 7
I am your son and lawful HEIR.
Game of Thrones Season 3, Episode 1
Heir to the Dreadfort and Winterfell.
Game of Thrones Season 5, Episode 6
And without an HEIR, well...
Game of Thrones Season 6, Episode 1
but he was the rightful HEIR
to the Seven Kingdoms.
Game of Thrones Season 1, Episode 7