If there was a 'no bill' the criminal case is over unless you can prove witnesses actually lied. Unfortunately, your best shot at justice is a civil suit for the assault which you could win. You need to hire an attorney for the civil suit.
The accused may even get there record cleared via an expungement after a grand jury "no bills" a case. The statute of limitations must run before the case is expunged.
A prosecutor can take the case back to the grand jury at any time before the statute of limitations expires or the case is expunged. There's nothing in the law to prevent it.
Regardless of what the ADA and the detective told you told you, if the ADA had wanted the kid indicted, the kid would have been indicted. It's very unusual for a Texas grand jury to no-bill an accused against the wishes of the prosecutor. More often a prosecutor will present a case to the grand jury but not ask for an indictment, so that he can blame the grand jury for not pursuing charges.
Without new evidence or political pressure on the elected DA (if it's Plano, it's Collin County, which is John Roach), it's not likely that the prosecutor will go back to the grand jury again.
Stab wounds are terrible injuries, and if the kid's family have enough money you may be able to get a personal injury lawyer to sue the kid on your son's behalf. The burden of proof in a personal-injury civil lawsuit is lower than that in a criminal trial.
Let me add that the civil statute of limitations is two years from the date of injury. That is, a civil lawsuit must be filed within two years of the stabbing or not at all.