Legally a person cannot be made ‘stateless’ - unless they have an alternate citizenship to fall back on, you cannot revoke theirs, no matter how reprehensible they are. The ISIS volunteers from Europe weren’t made stateless, just very dead or arrested
Doesn't really matter either way, he's not coming back, the way the war is going he'll probably be blown up at some point, and if he does survive he'll be arrested. His nationality is basically not relevant.