No you are not required to disclose any personal medical or mental health information to your employer.

On a practical level you might need to tell him more than "personal medical issue"----but its entirely up to you.

I would tell him it's something that you can manage, there just may be times where it demands more time away from work.

You do not want to turn this into a legal issue. Since you are not covered under FMLA your employer can make things very difficult for you without breaking the law. Keep things professional but friendly with your boss. Speaking from a managers perspective that will go much further then throwing the law in his face.