This situation sounds somewhat complicated due to the fact that so many kids are involved. It might be best to have your son evaluated by a mental health professional or the school counselor to rule out whether he is a victim of sexual abuse.
I agree with Mark, this is complicated. Experimentation, etc, at this age is not unusual. What bothers me is the outside influence of the older child. If this was just the two little boys - it would be not that unusual. I certainly would have a talk with your child about what is appropriate with your private parts.
And I think I would do all I can to avoid the older boy. I think that its probably important to find out who the older child was. Whether you bring a mental health professional or school counselor into the scene probably depends on how your son has reacted to the whole thing.
I agree with Sandman, the older children being involved doesn't sit well with me. I have a step-son who is currently in counseling because he was sexually abused by an adult. Going through the whole process of therapy, reporting the incident to child protection services, talking with the police and having our son do a forensic interview with their specialists I have been told a couple things over and over again about normal and abnormal childhood sexual exploration.
It is not considered in the realm of normal behavior when the children have an age gap or where coercion is involved. It is even less normal when you have both. Rather 6 and 8 sounds significant to you or me they are, they should be at completely different stages of sexual and social awareness and development. It also is usually one on one with a trusted friend who feels safe to them, not in a group setting and not because they are being told what to do, they do it because they are comfortable doing it and want to do it on their own.
Your son's part in this was totally normal. besides professionals I asked dozens of male family members and friends about boys touching each other, and every one of them acted like it was no big deal at all so the fact that he did it is not strange or abnormal. It's the fact that older children were involved and instigating that is weird and it's these older children you should be concerned both about and for. I would find out who they were and do some parental snooping, ask other parents if they have had issues with them, that sort of thing. If it's happened more than this one time I would report it. I may just report it even if I didn't end up hearing that it's happened more than once.