He is deceased.
Captain Freeze is an "Open-Source Character" and may be used by anyone. The only rule about using him is that your work must have this notation:
"Captain Freeze was created by Michael P. and Miguel Alejandro Neira Balbontín and is available for use by anyone, with only one condition: This paragraph must be included in any publication involving Captain Freeze in order that others may use these properties as they wish."
While his adventures, as well as his various artistic interpretations, are under copyright to their respective creators, the character and idea are free for anyone and everyone to use.