As you should. The flip-side of the coin is, what happens if/when things don't
work out? Would he really be comfortable knowing he could put you in the situation of working with an ex?
I like your philosophy of keeping work and play separate. It's an idea I subscribe to, to the point of not even friending workmates on Facebook! Relationships complicate professional matters, I'm sure your BF wouldn't be too keen on hooking up with one of his colleagues