As frustrating as this can be, your son's behavior is quite natural. If he's just gotten used to you and his father not being married, he might now fear that a stepparent will further complicate things and take up your time.
Why go out with someone who does not have your values?
If he's really put off by the idea of you dating a man other than his father, explain that you're making new friends, just like he does when he's in a new situation.
Try to maintain your usual routines with your son so that your dating doesn't disrupt his day-to-day life and he still has lots of time with you.
These feelings may be scary and overwhelming for him.
Talk with your son about how your dating makes him feel.