Kudos to your son! He seems to have found his passion, and an area of study which is a good "fit" for him.
Regarding "challenge", the word has a positive connotation in gifted education, for example it is used by the National Association for Gifted Children (NAGC) which provides this context:
Myth: Gifted Students Don’t Need Help; They’ll Do Fine On Their Own
Truth: Would you send a star athlete to train for the Olympics without a coach? Gifted students need guidance from well-trained teachers who challenge and support them in order to fully develop their abilities. Many gifted students may be so far ahead of their same-age peers that they know more than half of the grade-level curriculum before the school year begins. Their resulting boredom and frustration can lead to low achievement, despondency, or unhealthy work habits. The role of the teacher is crucial for spotting and nurturing talents in school.
Ideally, "challenge" means an opportunity for "stretching" and growth, learning new material rather than repeating what has already been learned and waiting for others to catch up. Appropriate challenge and support would provide an opportunity for being excited and interested by what they're learning and to have a sense of achievement... rather than "treading water" and experiencing underachievement.
