The reason your work buddy is pursuing you is that frequently boyfriends come and go. Your continued engagement with him about non-work matters clearly communicates your interest in him. It also unconsciously communicates your uninterest in your current boyfriend--do you talk about him (or the things you do together) much when you are with the work guy?

Plus, your newer comments about your blah relationship, well hell, looks like you are window shopping for boyfriends. Too bad you work together.