Welcome to BF!
The prevailing recommendation is to get some pure mineral oil, apply it liberally to the ivory, let it soak in for a few hours, wipe clean, then, if you like, apply a good quality wax, Ren-Wax is the best, but Flitz makes a good one too, do NOT confuse this with what Flitz is better known for, which is their metal polish, they do also make a quality wax, it is my belief that the wax seals in the oil and moisture which helps prevent the ivory from losing moisture over time and cracking or shrinking.
I have treated many stag and ivory handles this way and in several years, have not yet had a problem.
Good luck.