I would suggest just taking him to a show for experience of the sites & sounds
.Not to show but lead & ride him around the grounds,stand at horse trailer,be around other horses etc. It will be more like a training session for him. You can see how he responds to it all & your mom will probably feel less worried.
It will be a great learning for all involved & then you will know better if he is ready to handle actually participating in a show the next time