Is it possible that he's confusing knowledge with aptitude? In an adult, being 'good at' something is generally the result of putting in as much effort as one feels the need for, but a child has had a limited opportunity to, say, build robots. I always try to phrase comparisons like this as how fast a person learns, rather than the sum of their lifetime learning. I often point out to my kids that I may know more about X than they do, because I have decades of experience in it, but they know more about Y, because it's an interest they picked up before I did.
I have no doubt that for many, this is a big perfectionism problem - but from one conversation, it could be just a matter of terminology for the OP's DS. He naturally sees things he'd like to have mastered in his favorite topics, so he sees that he's mastered only a fraction of what there is to learn. Without considering the speed of his learning this feels the same as being 'bad at' the topic.
