Your first sentence isn't clear. What do you mean by "My son gave a 20 year old who beat up a kid for his sneakers?" Did your son know the 20 year old and had some kind of interaction with him, or was he just a witness to the fight?
When someone is charged with a conspiracy to commit a certain crime, he faces the same potential penalties as the person who actually committed the robbery. In this case, assuming that there were no weapons involved in the incident and no serious injuries, this would likely be charged as a robbery in the second degree, which is a violent crime and which has a minimum of 5 years and a maximum of 10 years of prison. With major crimes of violence, the law doesn't care all that much that it's not a first offense. Maybe the DA wouldn't offer a first offender the maximum on a plea, as that would be unduly harsh, but he would likely find some prison appropriate.
That said, if the judge only set $10,000 bail on a violent felony offense, then, in my experience, that's an indication that the state's evidence against your son may not be all that strong. If your son was merely an eye witness, then this may a case he should fight. He shouldn't have to settle for probation, even if the prosecutor offered that to him on a reduced charge. An experienced lawyer should be able to make a compelling case that your son was just an innocent bystander, and your son can choose to take the stand on his own behalf and point to his very real accomplishments, which he would have no need to jeopardize in order to help someone else obtain a pair of sneakers.