Hi Leapers,
That wasn't the best plan for your boyfriend to tell you that he was going to find it hard not to flirt with this girl, especially as he knows you're insecure.
Although it probably is not what you want to hear, I'd say he's holding on to you simply because he doesn't have the courage to end it, or maybe even stringing you along i.e.e keeping hold of you 'just in case'. The fact that you suspect he will go off with someone else you consider 'better' than you is either related to your insecurity, or because the evidence points to it. The fact that he tells you he wants to meet up with you more often and then proceeds to do nothing about it leads me to think that your suspicions of him may have some weight.
What is the nature of your 'pursuit' of him? Do you try to make contact with him regulary and fail? Talk to this guy and try to establish what's in his head.