W..
My personal opinion is that showing him pictures of people who look "sinister" isn't the way to go...... that way he won't expect his uncle, his coach etc.
Instead..... teach him that HE has control over what goes on with his body... in ANY capacity.... who he hugs goodbye, who he kisses, who helps him get dressed etc. This will teach him that NO ONE gets to touch him if it's not OK with him.
Now, this will backfire on you a bit..... when he uses it against you!!!! But stay strong. Your goal is to raise an independent, well-adjusted young man who has good self-esteem to say NO in the right circumstances.