Hi W., I agree with everyone who wrote in. Furthermore I completely agree with you and commend you for following your instinct even when it seems unfair that your other son had a friend over.
I live in a small town and allowed my son (14) to spend time in the sleepy town center with a 14yr old and 15yr old boy over the summer. I set some ground rules like never going into someone's house without calling and physically checking in with me at 5pm each day. He followed the rules. Well after the summer was over I found out that the two boys he was with consistently joined a larger group of much older teenagers and though I can't speak for the behavior of those two boys, I did find out that my son was the "try this" or "do that" kid that these older teens preyed on. It broke my heart to find this out and I have had major behavior problems ever since. Quite honestly I just don't know everything that transpired and I don't think I will ever find out. All I know is that my son changed and he stills wants to hang out with these older kids.
Needless to say, it was painful but I put a complete halt on any trips to town. Not only do parents have to worry about sexual predators but you also have to think about how vulnerable these younger children are. My son felt accepted and happy meanwhile they thought it was funny when he tried to smoke a cigarette and pot. Yeah real funny!
I have had my son in counseling for 4 months now. We monitor every friend. I call their parents to ensure that all activities are on the "up and up". Fortunately for us my son is really into sports so he plays in the fall, winter and spring. It was the summer that caught us and the older kids that brought my son into the group won't even look me in the face when I drive through town. They know that I know! (It also helps that my husband is a paramedic - wears a badge - and is part of the fire company.)
Stick to your instincts and don't allow any amount of pleading change your mind. This "guy" is too old to be any kind of playmate for your 14yr old. Even if he seems harmless, will you ever really know?
This is a big thing in a boys life. You are really brave to bring it to light. Thank you - for parents and their young sons.
H.