Since you admit that neither of you are stable enough to take care of your children, if he agrees, you could enter a written agrement and file it with the court that you be granted joint custody with visitation and parenting time determined by the guardians, that the court make no child support provision until dismissal of the current guardianship. there could be other approaches but that would seem to work.
The divorce and the parenting issues are dealt with separately by the court...sort of. You can certainly get the divorce and divide the marital assets and debts. If necessary, the court can order spousal maintenance and the like, too. With respect to your son, however, it seems unclear what the court will do with respect to him. The confusion for me is that you have stated that your grandparents have both legal guardianship and legal custody of your son. If your parents have legal custody, then any change in custody will likely need to be addressed in the legal proceedings that awarded your parents custody. If it is merely guardianship, then you can share legal custody of your son as the prior attorney noted until the guardianship is terminated.