If he pled guilty, under the eyes of the law and the judge he "did it." Acceptance of responsibly will likely impact how much time he does. His sentence will also depend on what his sentencing guideline scoring is. Do you have that info?
Armed robbery carries life or any term of years, and has a mandatory minimum of 1 year that the judge by statute cannot go below. In most cases the sentence would be determined by the scoring of the sentencing guidelines, which take into account the offender's past record if any, and which gives points for various factors relating to the crime, such as whether a firearm was discharged, or a person injured, etc.
The best way to get no time on the case is to get found not guilty. If he did not commit the crime then he should have a reasonable chance of achieving a not guilty verdict.
Contact me at 248-399-6930 for a free consultation. You and I do not have an attorney-client relationship formed by our communications on this website. Advice given by me on this website is general advice based on partial information. You should not rely on any advice given without first hiring a lawyer in the area where the case is pending, and providing that lawyer with full information.