@MerryD31 One option is to retire one of your playable Sims and then promote the son, but maybe you want to have all three Sims active at least for a while until they complete their stories.
How many playable Sim slots do you have? If you have two and want to add a thrird playable Sims, you will have to either buy the Sims slot with Sims cash or complete the roommate race quest to unlock a third Sims slot. The walkthrough is here:
https://answers.ea.com/t5/Game-Information/Roommate-Race-walkthrough-unlock-your-3rd-Sim/m-p/7920676#M21019 
