Son molested by an older child

My son spent the night away from me last night for the very first time, at his grandmothers house (my husbands mom) and her spouse had all of grandchildren staying the night also. My son just turned 6 and the other kids were all 5 and under except the one older one who is 10.5. My son had a blast and had so much fun with his grandmother. She sent me pictures and kept me updated the whole time. we FaceTimed before he went to sleep and he seemed to be having a great time. When she brought him home early this afternoon, right after she dropped him off, my son said “come in the room so I can tell you something” and when we got in there he immediately told me the 10.5 year old touched him on his penis and made my son touch his also. I started shaking I was so mad. My son said that he told him he didn’t want to do that but he said the older boy told him it’s a game called “the challenge” and if he doesn’t do it then he’s a girl and told him not to tell anyone ever. My son also said he was scared to keep saying no since the boy is older and bigger. He said it happened one time and then they were called to eat breakfast and the older boy told my son “we are gonna do this again after breakfast and anytime we see each other” (they only see each other a couple times a year and this was the first time they’ve ever been alone together before) but my son told me it didn’t happen again because after they ate they all got ready to drive the kids home. I feel so bad for my son that this happened to him and I don’t know where to go from here. We immediately contacted my husbands mom and told her and got the situation handled with the other boys parents and they all feel horrible about it and had a long talk with the boy. But I’m worried about how this will effect my son in the long run and I really need advice. Will it traumatize him in the long run or cause any issues? Do we need to take him to a therapist to talk about it? I’m not sure what to do because this is the first time anything like this happened with one of my kids. Anyone who has had a similar situation happen in their childhood.. did it go on to effect you or traumatize you in any way as you got older? or is it something you kind of just forget about? I’m so sad about this, But I’m so glad my son knew to tell me immediately. I’m proud of him for telling me instead of keeping it a secret and then have it end up happening again if he was ever with him alone in the future. The kid also has two very young sisters so I’m glad the parents are aware so they can protect their other kids.. because it most likely happened to them before if it happened to my son the very first time they were alone together. Thanks for any advice, I really need it.