Gosh I don't agree completely with some of the other responses you have gotten. I have often seen single mother/son relationships where the son doesn't develop properly to even function in intimate relationships because he didn't open his wings and fly.
Encouraging him to accept love and nurture from other relatives on his father's side is HEALTHY and you may have to give him a push.
I know men now who are in their 30's and all they do is work and spend the rest of the time with their moms, no social life, no woman, no friends.
I know another one who is now 21 suffered from severe depression cos he wouldn't go places, just wanted to sit at home and play videogames.
I know another one who is 7 and won't play much, just wants to sit at home after school and play video games.
Because he does know how much you love him he should trust that you're making a good decision for him in opening up to nurturing other relationships.
The longer you wait, the more impossible it will become in the long run and then you'll have a boy who can not allow other people into his heart and be a lonely person as a adult...
Amy