It happens constantly, and it is one of the more predictable problems in county jail environments.
Drugs inside jails and prisons are far more available than most people on the outside imagine. The demand is there because addiction does not stop at the facility door. People who were using heavily before they came in are still dealing with that dependency inside, and where there is demand, someone will try to meet it. Smuggling attempts come through visits, mail, staff, and people coming in from the outside on new charges.
Getting caught attempting to bring drugs into a facility is a serious matter. It typically results in new criminal charges on top of whatever your son is already facing, which is the worst possible position to be in while already locked up. The evidence standard varies, but facilities take these charges seriously and prosecutors tend to pursue them aggressively because contraband inside a jail creates safety problems for everyone, including staff.
The fact that there is no solid evidence in his case is the most important detail right now. That is exactly what his attorney needs to be working with. A charge without strong evidence is a charge that can potentially be beaten or negotiated down, but that outcome requires legal representation that knows how to challenge what the facility is claiming.
Get an attorney on this immediately if one is not already involved. A new charge while incarcerated can significantly affect his current case, his housing status inside, and his overall sentence exposure.