You need to use boundaries. What your boyfriend is doing is inconsiderate and disrespectful. However, rather than create drama, [i]calmly and firmly [/i]tell him that you can no longer take care of his son unless he’s given you 24 hours notice and you’ve agreed ahead of time to watch his son. Tell him that if he brings the boy over without getting your okay first, then you just won’t be able to take him. And….you have to stick to that rule.
If he continues to show up with his son without warning, then you have to say, with a smile on your face — “I’m so sorry, but I already have plans today, and I can’t take care of your son! I’d love to see him when I’m free — call me in advance next time so we can all be on the same page!” Then shut the door.
If he’s a good guy, he’ll get it and change his behavior. If he’s not a good guy, and gets resentful then you’ve figured out that this is not someone with whom you should move forward in a relationship. It’s not fair to you, your son, or his son to continue the dynamic you’ve both been engaged in.
I know this is going to be a little tough for you but if you don’t do it, things are going to get worse.
Good luck! 🙂