5 year old touching other boys

Is it normal that my 5 yr old son asked a 4 yr old boy to pull his pants down? What type of discipline should be used, if any? My fiance is the father of the 4 yr old and he seems to think my son has a lot of issues and needs help because he laid on the 4 yr old's stomach and was pretending like he was his "girlfriend" and then the next day he apparantly asked him to pull his pants down. My son said the 4 yr old initiated it but my fiance can't believe that. He said my son is lying and he's not going to put up with this type of behavior. My son could be lying, but how should I handle this if he is?