What if the Nega has retained its appearance or has spread at the end of the second week?
Rashi: If the Nega has retained its appearance or has spread at the end of the second week, the Kohen declares the Metzora Tamei. 1
Ramban, Da'as Zekenim (both citing Nega'im, 1:3), Hadar Zekenim citing Megilah 8b, and Rashi there): As long as the Nega did not spread, it is Tahor. 2 Moshav Zekenim - R. Shimshon (Nega'im 1:3 DH she'Kol) derived so from Toras Kohanim.
As implied by the words of the Pasuk.
Ramban: Even it went from white ke'Sid ha'Heichal to white like snow. And the reason the Torah writes "Kehah" and not 'Amad be'Einav' is to teach us that even if the Nega became dimmer, if it has spread, it is Tamei. Da'as Zekenim - or, Kehah teaches that even if it became dimmer, we do not consider it a new Nega.
Seeing as the Kohen declares him Tahor, why does he need to immerse his clothes?
Rashi: Since he needed Hesger, he is considered Tamei and requires Tevilah (he and his clothes).