His radio is faulty!
Radios of all types may pretty much be left on a frequency, & not drift much at all.
If you are listening to an SSB signal & have nicely "tweaked" it in so the speech sounds normal, it may well drift a bit over a reasonably short time & need some adjustment, but "drifting all over the place" is not the hallmark of a properly designed radio in normal operation.
Many FM Broadcast radios have "automatic frequency controls" to keep them on frequency, but this is more a "nicety" to ensure that the stereo pilot is resolved correctly to ensure decoding works.
In an ordinary mono FM radio, any drift would have minimal effect------ after all, the channel width is " as broad as a bull's bum" at a couple of hundred kHz wide.
If, however, the AFC itself is faulty, that is a possible cause of your problem, as it may be over correcting wildly.
If you are referring to Medium Wave AM broadcasting, drift should be of even less consequence.
Many thousands of such radios from the tube era sat on shelves in Workshops, tuned to the same station for decades.
They might have drifted enough to sound a bit "muffled" after years, but nothing on the scale you are referring to.
Of course, you could tell your brother to switch "scanning" off!