I’m not an expert, but my son’s doctor has been watching for signs of early puberty and she said that darkening (the testes in my son’s case) wasn’t really what they look for, but a change in size of the testes.  Even if your son isn’t affected, he may have late onset, or carriers can have symptoms so it’s probably worth checking out.  Also, usually adrenarche - or adult body odor, pubic hair or acne would come before changes in genitals.  But I’d let your other child’s endocrinologist check him out anyway.