Family transport for an inmate transfer is not standard practice and has not been something commonly granted at federal camps, but the situation at minimum security camps is unique enough that it is worth asking directly.
Federal prison camps house the lowest security level inmates in the Bureau of Prisons system, and that classification comes with privileges that do not exist at higher security levels. One of those is the possibility of unescorted travel for transfers. Rather than being transported in a prison van with officers, some camp inmates are allowed to travel independently to their next location, typically via Greyhound bus. The camp has a designated driver who takes inmates to and from the bus station or other release points for exactly this purpose.
Whether that unescorted travel can happen with a family member providing the transportation rather than the Greyhound bus is a question worth putting directly to the camp counselor at Herlong. Federal camp counselors are generally accessible and straightforward to deal with, and this is exactly the kind of question they can answer definitively. The worst they can say is no, and if the answer is yes it saves your son a bus trip and gives your family a meaningful moment in what has been a difficult period.
Call Herlong Federal Prison Camp and ask to speak with your son's counselor. Explain the situation, that he is awaiting an RDAP transfer and you would like to know whether family transport is a possibility. Let them guide you from there.
In the meantime, the RDAP transfer itself is the priority. Encourage your son to stay engaged with his counselor about the timeline and keep his record clean through the wait.