Unfortunately, if a death occurs during the commission of a felony, all participants in that felony can get charged with and possibly convicted of felony murder. The felony murder statue applies whether the homicide was intentional or accidental and whether all the co-defendants knew a homicide was going to happen or not.
If your son was convicted after trial and believed that his sentence was unduly harsh or that there were other injustices during the course of his trial which resulted in his conviction, his remedy would have been to appeal his conviction. There is only a limited opportunity to appeal on most grounds, so if he hasn't done that in 7 years, most avenues for appeal are no longer open to him and those few that remain do not seem to be applicable in your son's case from what information you have given me.
If your son is serving time because he took a plea bargain, it's very difficult to contest the sentence because he agreed to it in the first place. The record of his taking the plea could be used against him to show that at the time he freely admitted his guilt and voluntarily entered into that agreement after discussing his options with counsel. That would likely cause any motion to reconsider his sentence to be denied.
I am seeing only the very tip of the iceberg, so I can only speak generally based on limited information and alert you to the problems he'd be facing having waited 7 years to address this issue. If you want to do everything you can, however, you might want to consult with a local post-conviction (appellate) lawyer to see if there are any viable grounds to reopen his case at this time. Many lawyers have free or low cost initial consultations.
Otherwise, if he is eligible for parole, his options would be to work with a parole lawyer so that he could have the best chance of being released when he's first eligible. Or he could apply for clemency from the governor. Here's the clemency application form.