he says he misses his friend from regular class (autistic friend) and would rather be bored in regular class with his friend than bored in gifted class with kids he doesn't know.
Sounds pretty straightforward to me! Just because a class is "for gifted kids", that doesn't necessarily mean it's advanced enough for him. The school's idea of gifted is usually not as "out there" as most of us here really need. And if you're going to be bored, it certainly seems like it would be less irritating to do it with someone you know.
On the other hand, "bored" can be a code word for any number of things, and it wouldn't hurt to talk to the teacher and see what's really going on -- ask what your DS does in the class, mention that he doesn't seem to be connecting with it, and is there anything you can do to help him get more out of it. The boredom/hatred stories about school that I get out of my DS do not always have deep roots, or any at all, in the truth, so I use a lot of "what's going on with..." and "what do we need to do..." in talking with his teachers.
