My son has a pigmented mark on his upper back, please advise?

Hello — this question pertains to my son, who is 19 years old. He has had a pigmented mark on his upper back which I first noticed when he was about 8 or 10 years old. It started off looking like a cafe au lait spot, and throughout the years, it has darkened and become rougher in appearance. It is a little smaller that an inch, and looks like some smaller spots grouped together. His pediatrician had never raised any concern when I pointed it out, and he has no diagnosed medical conditions, I recently took him to a dermatologist, and the physician’s assistance there examined it with a dermascope and said it was a “linear epidermal nevus”, and further said that it can simply be monitored yearly and that the chances of it becoming anything dangerous are slim to none. Unfortunately I made the big mistake of googling epidermal nevi and ran across information concerning related syndromes associated with epidermal nevi, and also information indicating that these nevi could have a genetic mutation with associated skin disease in offspring. Again, my son has one solitary epidermal nevus and is otherwise healthy; just looking for some reassurance that I should not be concerned at all about related syndromes and potential gene mutations in his offspring (Dr. Google can be a dangerous man!) Thank you!