I would find out what's customary on your campus. On my campus, people made paddles (with their own letters) for their pledge husbands/wives, or for someone else with whom they had some sort of close friendship related to Greek-ness, i.e. they were PHC/IFC counterparts or something like that. Usually, though, they were close friends, not bf/gf things. Would you still want him having your letters if you broke up?