I have taken various antidepressant and antianxiety medicine over 30 or so years. I've not taken Xanax but I've known those who have. None of them complained about any side effects. I've taken Lexapro and as I remember it didn't help me and so the doctor switched me to something else.
Nearly all of them can cause sleepiness or a tired feeling at first until the body gets used to them. I've not been addicted to any that I've taken and have not heard from the medical world that addiction is a concern. I do know that when one wants to stop taking them they have to taper off gradually or they will experience some serious side effects.
I'm in favor of using anti-anxiety and anti-depression medication. They have helped me immensely. I understand your nervousness about them since you've had not experience with them. Give them a try. If they don't work, try a different one. There will be no permanent results from using any of them as long as your son takes them under his doctor's supervision. People run into trouble when they make changes on their own.
You can find a list of possible side effects on line by googling the drug. Please remember that those how have most of the side effects are in the minority. Report anything that causes you or your son concern and be assured that your doctor and/or pharmacist can help you with them.