The new gun charge alone carries a mandatory minimum of 5 years for felony possession of a firearm. That minimum applies regardless of circumstances, criminal history, or anything a judge might otherwise want to consider. The law requires it.
The prior Class X conviction makes this significantly worse. Class X offenses are serious felonies, and having that history on record affects how the new charge is weighted at sentencing. Depending on the state and the specific firearms statute, a prior violent or serious felony can trigger enhanced mandatory minimums that push well beyond the standard 5 years, sometimes to 10 or 15 depending on the charges.
On top of the new case, he is also facing a parole violation. Being arrested on a new felony charge while on parole means his parole is almost certainly going to be revoked, which means he will serve whatever remaining time was left on the original sentence in addition to the new sentence. Whether those run concurrently or consecutively is one of the most important outcomes his attorney needs to fight for.
Concurrent means both sentences count down at the same time. Consecutive means he finishes one before starting the other. In a situation like this, the difference could be several years of actual time served.
Get him an attorney immediately if he does not already have one. The interplay of the parole violation, the prior Class X, and the mandatory minimum on the new charge needs someone who understands all three simultaneously.