You could always buy a seachem purigen pad for your hospital tank, this should prevent the need for cycling for the duration of his quarantine.
Basically all you can do is keep him in the best water conditions available, keep feeding him pea but don't over do it, treat with an anti bac and look out for any other symptoms or oddities however small. Maybe make a diy tank divider and give him plenty of cover to help him avoid any stress if kept in the community.