OK, this might sound really stupid and I apologize if it does, lol!
But the first thing I thought of after reading Shell's post (which I agree with) was this, and this pertains to him maybe not understanding that you can love both of them at the same time. I'm wondering if he has a pet? And maybe if he did this might help him understand what it means to love more than one person at a time?
Maybe I am way off base here, lol...but it was just the first thing that popped into my head.
Good luck with everything!
The first thought that jumped into my head is whether or not your son is really scared that this man will do these things, or is he scared of you eventually getting married and this is the way that emotion is manifesting itself.
We had a kind of similar situation like this in my family. My sister married a wonderful man 3 years ago. Her husband has a son, M, who is now 13, who they have full custody of. (Her husbands ex had all sorts of substance issues and has not been a constant presence.) My sister, her husband and his son lived together before they married and it went really well. They got a bigger apartment and M had his own room for the first time in his life. He LOVED having a space of his own and they were all really happy. However, when my sister and her husband told him they were getting married all hell broke loose. It was only through family therapy that they eventually go to the root of it. M was afraid that marriage would lead to a baby and that the baby would be given his room. It wasn't that he didn't love my sister, he just made some erroneous assumptions about life after marriage. (Of course they eventually bought a house, had a baby, who my nephew adores, and my nephew has a huge room he doesn't have to share!)
Sometimes what the child says is the issue is really masking a deeper issue. I wish I had some practical advice to give you, this is a tough one.
I think if he can make you to believe this man will hurt you both
it will be back to you and him.
i think he see's your friend has a threat to what you and your son have
try to make him see that you an love more than person at a time
and loving someone else will not stop you loving him
love shell