Wesker is most certainly dead. After violently mutating, and destroying his body, Wesker was not only melted in lava, but also shot in the head with two simultaneous rockets, most likely destroying all of his remains. Besides, it is believed that he is suceptable to being killed by a headshot, and both rockets penetrated both of his eyes, which then blew up. The ability to clone him from his remains, so that he may return in sequels is very unlikely, as the heat of the volcano would have burned away any trace.