You seem to have a handle on the most likely reasons:

• One interpretation is that he's insecure and needs validation.
• Another is that it's a power move to keep you in line by letting you know he has options so don't step out of line, or he'll access those other options.
• He may just be egotistical and likes to brag.

Next time he brings it up, just calmly ask him "I'm not sure why you are telling me this. What am I supposed to do with that information?"

You could also go with "yeah, I get a lot of offers too" and when he gets upset, respond with "oh, I thought we were sharing."

No matter how you slice it, it's a douchy maneuver.