My boyfriend stays in contact with the son of his ex and I don't want him to.
I don't get it. What makes you think he should cut contact with his ex's son?
The kid isn't even his. Why would he stay in contact?
If you actually loved your boyfriend then you would never give him this type of ultimatum. There's nothing wrong with him staying in contact with his ex's son if they have a good relationship, and his ex is okay with it. Your actions have consequences and nothing good can come from dictating what your boyfriend does.
I should have full control over someone I'm dating.
No, there's no reason to think that you should have control over someone you're dating. Your boyfriend is his own person, and it's wrong for you to tell him what he can and cannot do.