At a time when he isn’t panicky sit down and talk to him about what leaving means. Show him examples of things he leaves every day but that he never forgets about. Like his favorite toys, pets, etc. Then explain to him that you would never forget him and will always be back.
Also if you have begun “slipping” out unnoticed to avoid the scene, stop it. Yes, you may make the occasional stress free (for you) escape but you are training the child to not trust you and to always keep his eyes on you.
Instead, in a calm and in control voice say “Johnny, Mommy is going now and I’ll be back at ____insert some recognizable time for him like “after nap, after snack time, story time, etc. Then hug him and walk out…and keep walking no matter what.