Jenn, FEV is a test having to do with exhaling, and in my case there's a "1", meaning in one second; FEV1. It is measured in volume and also percentage. In reading about it online, it really has more to do with the lung itself working (emphasema or other lung problems; COPD) rather than the muscles which we're really concerned with as ALS patients. Your dad's score doesn't make sense. It may have been 48%. Or possibly 4.80 liters. These would be very different sounding scores, the first not so good, the second great.
The test isn't one which is very pertinent to ALS, but would be to another type of lung problem, so unless your dad has COPD, I wouldn't be terribly worried. You can call and ask their office (Pulmonary) to verify and explain the score.
If anyone knows differently, please feel free to correct me on this.
Ann