I've had my bearded dragon for almost 2 years now and he has always had at least some stress marks. Most of the time they are just feint blacks lines on his belly and neck but i'm not sure i've ever seen them go away. I provide him with everything he needs, he always seems happy and is always comfortable with me, could this be permanent?
It's possible that it's just natural scale colors. If you could post photos of him it would be helpful. You can upload photos here https://www.beardeddragon.org/useruploads/ and post them to your thread using the XIMG button when you reply.
If you would also like to provide details about your setup for him, we'll be glad to review it to make sure everything is just right for your buddy.