Do you still help him bath, like help wash his hair and back? I always started this topic first as a funny thing like saying: I helped make your back and your head...laugh a little and explain how wonderful you made him. That only you can touch him because you are his mommy and you made him.
Wash the bottoms of his feet and tickle a little, ears, nose, chin and repeat, I made these and only I and you (him) can touch them and no one else can or should. Keep it very light hearted and fun and then when he is laughing and having fun ask him if he understands.
I did it like this with all 4 of my kids and they are very aware of strangers and the bad touches they can do. Start when they are young and you have an influence!
Funny story, yesterday at Target, in my honda civic, 14 yr old dd is sitting half way on my wallet. I reach over and tug it out from under her cheek. She says straight faced..."Uuuhhmm, I would appreicate it if next time you asked permission before you touch my bottom!! Then she smiled...

I laughed so hard... and asked her where do you come up with stuff like that so fast!

She said..."I just thought it was funny to say!