Wow, that's a sensitive subject. Do you have enough rapport w/the friend's parents so that they know it's not true even w/o checking?? I hope so.
I know it must be hard. You might think about having him evaluated by a counselor to see if there are any underlying things that are bothering him.
I taught fourth grade (that age) for quite awhile, and this is actually kind of common-it happened many times with students. So much so that we developed a slogan, "I won't believe what they tell me about you, if you don't believe what they tell you about me!" Of course, that's kind of in gest, but it's true. I am sure you all are wonderful parents. Often times it's an attention thing, not to say you aren't paying enough attention as it is, and I am not sure just what to recommend. And, although I said couseling might help, it would require your son to be honest and open up, which I am unsure that he would do given the facts. They might uncover that he feel anxious of maybe it IS just his friend influencing him.
Another question....would there be a way for you to get him "away" from this friend? At this age, they become VERY swayed by friends, probably for the first time. I would see children who switched classes to "get away" from a child the parents thought was causing their child to act abnormal. Often times, IT WORKED! Do they go to the same school, activities, etc.?? So, that's another idea.
I wish you the best of luck!!!