Rashi (98b, DH "Ain Mashiach") explains that he believed in Mashiach, but mistakenly thought that all of the Pesukim discussing Mashiach referred to Chizkiya who had died. Rashi (99a, DH "Ain") later explains that this did not mean that he did not believe the Jews would be redeemed, but rather that Hash-m would redeem them by Himself without sending a Mashiach.

All the best,

Yaakov Montrose