My son 16 years suddenly said that his vision is blurred and could not see from both the eyes. The doctor has asked to orbit MRI scan. Is it serious?

Difficult to predict. There could be an inflammatory condition, glaucoma, vitamin deficiency, etc. But could also merely involve transient chemical/metabolic imbalance, medication reaction, etc. Definitely get imaging asap, and see an ophthalmologist soon.
Serious. If your son is experiencing bilateral vision loss or partial vision loss, and his ophthalmologist is ordering a orbital mri, that's serious. Not normally ordered for anyone with blurred vision unless an orbital issue is suspected. Speak with his eye doctor about this.
It could be? If the scan is negative, some times it is psychological at this age to get more attention.