Hey there, Joy, and welcome to planDisney! Thanks so much for your question!
Oogie Boogie Bash - A Disney Halloween Party is a lot of fun, and September 17 is going to be a great night at Disney California Adventure! I can absolutely understand wanting to get your grandson in on the fun.
Unfortunately, it looks like tickets for the September 17 date of Oogie Boogie Bash have sold out, along with most of the other dates of the event. This event is incredibly popular and individual nights tend to go quickly, so that date being sold out is not a surprise, though I know that's not what you were hoping to hear.
While ticket sales are final and you're not able to switch the dates of your already purchased tickets, as of the time I'm answering your question, it looks like there are still event tickets available on September 8th. If you decided to visit on an additional date with your grandson then it looks like this would be your last option - though I imagine these tickets will sell out quickly!
Thank you again for stopping by with your question, Joy - I do hope you have a wonderful time on your visit to experience the Oogie Boogie Bash in a few weeks.
Have a Magical Day,
Greg