ADD medication and Fluoxetine for OCD and anxiety

My adult son is on the ASD spectrum and has been taking Fluoxetine for a few years. It made a huge difference to his OCD tendencies and anxiety. He has recently been diagnosed with ADHD/ADD and is being medicated with a stimulant medication which is helping him at work. He dropped the Fluoxetine, fearing Serotonin issues but has now reverted to a state of heightened anxiety and OCD. Does anyone have experience with taking both medications together?