Could my 5yr old son been molested?

My 5 yr old son is displaying (what I believe to be) too much affection towards his 9 yr old friend which is my roommates son. He has been caught kissing him on the lips, laying on him and fondling the other boy's private area. We have spoken with both boys and each time we think the situation is under control, another incident happens. My concern is that my son may have either learned these actions by witnessing others, or worse, something happened to him while in his grandparents care. When asked, my son just keeps saying that nothing happened to him, and that he just made it up, but I just can't seem to fathom that a 5yr old could make up these acts out of nowhere. He hasn't been around other children and begins Kindergarten this year, so I know that he hasn't picked up bad habits from other kids. My daughter who is 8, also shows some signs that concern me, but nowhere as extreme as my son has shown. Since my children have been away from their grandparents, they no longer withdraw or act out violently, just now I am seeing them acting out very maturely which makes me wonder what has happened around them or to them. Any advice on how to handle a situation like this? How do I talk to my son so that he Truly understands that his actions are wrong? I am wanting to take both my children to seek help, but if nothing happened to them, I am worried it may cause more harm than good. Is there anyway I can help my children open up to tell me the truth so that I can figure out what's the best course of action to take? Or should I just take them to a therapist regardless and hope it helps? Thank you