Can Teenagers Have Separation Anxiety Disorder?
“Can you have separation anxiety disorder in adolescence? My 15-year-old is acting very strange and clingy all of a sudden”

If your teenager is acting ‘strange’ it may mean that there is something going on with him. I think that it is definitely a good idea to have a chat with him to see what is going on. Tell him that if he does not want to talk to you, he could talk to another trusted adult in his life or perhaps he would agree to talk to a therapist. If he goes to school, have a chat with his teachers to see if there is anything going on at school that you may not know about.
You mention separation anxiety disorder and yes, adolescents can have it. Teenagers living with separation anxiety show signs of fear when they have to say goodbye to their parents or caregivers or just the thought of being separated from them. This anxiety can make them miss school or activities that require being away from their parents or caregivers.
I would urge you to talk to your teenager. If you are worried about him, do take him to a mental health professional.
You may find these articles useful:
What Are Your Thoughts on Anxiety Medication for Teens?
How to Help Children with their Anxiety?
How to Help a Child with Anxiety?
This masterclass may also be useful:
How to Have Difficult Conversations with your Teenager
I hope this information helps. I wish you teenager all the very best,
Love,
Ana