12 year old boy touching 7 year old boys

need some feedback on this big situation I am in, my 7 year old boy, who i adore, has been sleeping over with my boyfriends children, this has been happening for 3 years and never had a problem.
Last night my son informed me that he doesnt want to sleep because the 12 year old boy has been touching him,
I have asked him some delicate questions and he has told me that  it has happend the last few times he has slept there approx 3 times, he feels him down below and the bottom area. My son has been asleep and woke up to the scenario, Naturally I am devasted and in shock.
my partner is equally upset, any advice please!!!!!

[margypops]

No more sleep overs ...and the 12 year old has to be spoken firmly to and  needs counseling .It very often is a learned behavior so possible some has done the same to him ...best to leave sleep overs alone this happens a lot young children should be at home ..