It looks like there is a reasonably accessible pommel nut. I reckon you could unscrew the nut, remove the pommel, and see what's going on with the handle. Often times, these older stacked handles have leather washers that dry out and shrink over time, leading to gaps in the materials and loosening up the handle. You might be able to add an extra leather washer to fill up any gaps in the handle construction and re-tighten the pommel and nut back into place. This might not work if the antler panels come off in multiple pieces, rather than as one piece. If you do this, please take care not to scratch or rip up the brass pommel nut or the aluminum handle (which is easier to do than you might think).
If you intend to make this knife into a user then it may be worth it to try to modify the knife. If, however, this is a sentimental piece that you want to preserve then maybe it's best to just leave it as is and not risk causing any damage.
Somebody may very well come along with better advice, so wait a bit to read some competing suggestions before moving forward.