Throwaway but

A colleague from another department has been getting closer to me. We worked together on a project previously and we had a good time. He's pretty cute and interesting, but our relationship is platonic and revolves around work and small talk. Our conversations are mostly superficial and we don't text about our personal lives, but we get along really really well. I think he is interested in me and he asked me out for dinner as he is leaving the company soon. He doesn't know that I have a boyfriend. How do I break it to him? I don't want to lose this friendship though and I think once he knows I have a boyfriend he would probably distance himself and I'll lose a good friend. Also, I know I'll be blasted for this but this whole scenario comes at a very complicated time as I'm going to be breaking up with my current boyfriend as our relationship is going no where (decided even before these events). I'm being very careful because I respect my current boyfriend and I'm not going to cheat, physically or emotionally, even though we'll break up soon. However, in another world, the guy from is someone I can see myself dating. Any advice?