You should have a casual conversation with your son about his private parts and his brothers and that they shouldnt touch each others. Explain to him if he is touching himself that he must do it in private and not infront of anyone as it is a private thing.
Just to reassure you this is very normal for him to masterbate at this age.
My daughter found her privates at the age of 2 and hasnt stopped since. She is 7 as well.
I dont want to make her feel ashamed of her body but I did however tell her she must do it in her room and no where else and not infront of anyone. I think for my daughter she does it because she has anxiety and it relaxes her.
It's not uncommon for small kids to masturbate, surprisingly even babies sometimes figure it out, though it's anyone's guess what it means to a kid before he or she goes through puberty. That said, puberty is coming on faster and faster for American kids. Little girls are beginning to get their periods at 8 and 9. (I've read that it has to do with better nutrition.) Anyway, see a pediatrician if you are really concerned.
dont l them shower together anymore you should be worried he should not know how to masterB.
It's probably just kids playing, but since the 7-year-old is becoming aware that his penis is something more than a hilarious body part to play with, you might gently discourage that kind of goofing around. My son learned the concepts of "private" and "public" relating to behavior in restaurants and grocery stores when he was about 3, and now it is easy for him to understand about "private" as it relates to body parts. Your 7-year-old can be told that while he can touch his own private body parts, and a doctor can or a parent can when washing him, he is not to touch others' private parts.