When they say work they don't mean casual riding do they? We have been riding Okie at a walk and very slow lunging just to start getting him conditioned again. Not long or intense though.
Ask your vet perhaps, and we'll see if the others on the thread have actually been through strangles.... But I wouldn't work him if he has any symptoms still... Can he breathe? The symptom that he's not ready would be continued discharge and getting easily winded. You want to be careful, horses who get very sick with respiratory ailments have a hard time getting their wind back, you don't want to do any permanent damage.