Why did he say this? I told him I was seeing someone else?

Because he probably likes you but can't tell you and it pi$$ed him off for you to see someone else. And he probably does not want you seeing someone else to start with but feels that it is not proper to tell you. And why would you tell each other if you are going to (or have had) with someone else.? You really need to find out how he really feels about you before you screw it up.