This is a difficult situation with serious exposure on multiple fronts. A new gun charge while on probation for a gun offense creates two simultaneous problems: a probation violation on the original UUW case and a new criminal prosecution on the second charge.
If the new case is charged federally as a felon in possession of a firearm, the mandatory minimum is five years with no possibility of parole. Federal gun sentences are served at 85 percent and the mandatory minimums are real. There is no judge's discretion to go below five years on a federal felon-in-possession charge.
On the probation side, the original UUW probation is almost certainly going to be revoked, meaning he will likely serve some or all of the original sentence alongside whatever comes from the new case. Whether those sentences run concurrently or consecutively matters enormously and is something his attorney needs to fight hard on.
The most important thing right now is competent legal representation. A new gun charge on top of an active gun probation is not a situation where a stretched public defender caseload produces the best outcomes. If there is any way to retain a qualified criminal defense attorney who handles gun cases, this is the moment to do it.