Kevin felt a sudden chill when he was fighting the werewolf monsters. The monsters had a bigger build than his, but he could move so agile and managed to strike a few monsters with his ice attack.
It was all his plan. He didn't know what happened, but even after the antidote was extracted from his body, he had the feeling that his powers were still in his body.
That one time when his palm got hurt when he tried to save Jack from Julia, his wound was immediately healed even though he didn't go to the hospital. He suspected it, but he didn't tell anyone including Kris.
A few days after the incident, he secretly tried to use his powers again, and he guessed it right. He indeed couldn't use his powers again the day after the extraction, but then the powers were there again inside his body. He could use his ice powers again, and he also could use the healing powers for himself.