I was so happy to see Erinn's response, because I had the same thought.
I am a 7th grade teacher, and without any more details regarding the events, I can think of all sorts of explanations for what your son is experiencing, and only one of those explanations would be playing favorites.
One of my first thoughts was that the other student (or students) involved may have IEP [Individual Educational Plans] that are required by law for students with disabilities who require special accommodations and/or modifications.
For example, if I had a student who had special modifications on homework (perhaps doing a limited set of the homework problems), I could certainly see other students thinking I'm playing favorites by giving him full credit for partially-completed homework assignments.
I'd also say that it's quite common for middle schoolers to be particularly concerned about fairness -- and possibly seeing favoritism in just the relationship a teacher may already have with students that the teacher knows from last year.
But teachers are also human, and I know I do get along better with some students than with others. I try very hard not to let that show, as does every other teacher I know. I've also been in classes with teachers who played favorites (and occasionally maybe even benefited from a bit of favoritism.)
Without any specifics of what is actually happening, however, I would probably give things a few weeks to settle down and then reassess. I would probably try to listen to my child's complaints without encouraging them in any way [Responding like "I can see why you would think that is unfair" as opposed to something that might encourage the belief that the teacher is playing favorites--something like "That teacher's behavior is simply unacceptable. I will not stand for that!"]
As others have suggested, though, if you really feel the matter can't wait, you should approach the teacher with an open mind. He or she may have some insights to share about the situation that will change your perspective.
I don't say all of this because I assume the teacher is right, only that it is far more helpful to go in with the approach that you both want what is best for your child, and a teacher has a different point of view on the situation than either you or your son.