Is he miming, playing along to a backing track? Or ifhe is playing, how can that be?
Wireless radio transmitter, attached to the strap or on his body somewhere. At the other end is a receiver plugged into his amp on stage.
Random Google of a range from a music store... https://www.gear4music.com/Electric_Guitar/Wireless.html
Equally random image of both halves of an example...
The systems available in 1979 had a rather unfortunate issue of being very near the same frequencies used by taxi companies to call their drivers.
It was a fairly common interruption in those days, halfway through your best solo effort.. "Car 39, come in. Car 39, can you do a pick up at the airport?"
I don't remember where I first heard this but, Angus Young was an early adopter of wireless and apparently liked the tone the early ones produced even using it during studio recording.
This page has a overview: