If an asthmatic patient has an o2 blood saturation 85% does that mean he should be in the emergency room or not?

Anyone. Asthma or not with saturation of 85% would likely be in the er except a few of chronically known hypoxic patients in whom everything is done. More so in asthma because the hypoxia in asthma is a later occurrence, they compensate initially by breathing harder and to be hypoxic means it is harder to keep up with the body's need. So, yes, go to the er.
Low pulse ox. Yes if you are having an asthma attack, and a low o2 saturation ( normal 95 or higher), you should be seen by a physician right away.