[@thewizardguy] I guess massive supervision it is. If he's immune by simply being around it and not by its actual effects on him then it would be increasingly difficult to restrain him. Better keep Bambietta close then. That or some of the upcoming characters joining the group. In any case if he's at least reasonable there's no need to restrain him that much. 

Though the aliens aren't in OOC. The best idea would be that they'd simply place a dormant Chryssalid larva (because they don't want to hinder him) in his brain or nervous system. Once active (when he gets out of line) it'll likely limit him for a short period of time and either he'll be immune to its effects quick enough and the Chryssalid dies or it pops out through rotting flesh (zombie stuff) and he eventually regenerates.