Thank you for the clarification.
Of course, I don't know all the specifics in dealing with your son's case and/or the prosecutor involved in that case. That being said, I have never heard of a defendant being held accountable because of another person's record.
Again, I obviously wasn't present at this conversation between your son's lawyer and the prosecuting attorney. However, it is not uncommon for prosecutors to offer different plea agreement
to two people involved in the same event if there is a big disparity in their record information. Your son's lawyer may be telling him exactly what the prosecutor said, but it sounds unusual to me.
If you or your son have the financial means to meet with another lawyer for a consultation (sometimes, they are free for an initial consult), I would recommend doing so. A local lawyer will possibly know this prosecutor's reputation and can tell you if this is common practice for that particular prosecutor. Also, just for the few facts you've given me, it doesn't sound like the prosecution would have a very strong case against your son. (That, of course, depends on many factors, but as an accomplice, there is less evidence from the start to convict). If the prosecution's case is weak, that is even more incentive for him/her to try to strike a deal.