When your son’s interest in Spider-Man interferes with school and his daily schedule it is time to set limits. Talk to your son about how everyone has interests. If you are a golfer, you can’t talk about golf all the time, bring your clubs to work or practice your swing during business meetings. There is a time and a place for hobbies.
The classroom is NOT the time and place for Spider-Man. Spidey can surface at recess. But even then, Spider-Man cannot be around all the time at recess; sometimes Peter Parker has to be the role he plays in order to live a more normal life. During school, your son needs to be “normal” and focused on school.
Perhaps it would be helpful to introduce him to some other superheroes such as the Hulk or Superman since they have qualities a 6-year-old can admire. If you can, purchase plastic figures of these superheroes for each of his friends to use when they come over to play superheroes.