I have to agree with margy. unfortunately this sounds like a learned behavior...being curious is one thing, but trying to force other kids to let him touch/kiss them, exposing himself, this is not normal curiosity....
I think its time to take an in-depth look at his life up until now and try to figure out if anyone could've done something to him...and get him into counseling to help sort this out before it becomes a serious problem.
Please do NOT ignore this. If, heaven forbid, something has happened to him, get him help.
There's the possibility that this is just some behavior that he latched on to, like you said, to get attention...but my gut is screaming something more is going on, so I urge you to get him to a professional for an evaluation. If nothing comes up, good! You can deal with it behaviorally. However, better safe than sorry.
Is it possible that he has seen this behavior from someone in the family? children usually learn from others ,, you say he has an older brother could any of the behavior come from him or his sisters ,also cousins..I suggest you check out the dynamics Possibly some counseling may bring to light where he has seen the behavior .....