Unless you need to be legally precise I'd just call him "Cousin"; I even call my cousin's children "Cousin". I just wish there was a name for my daughter's husband's parents.
No, it's just clumsy when we refer to them. It's even worse now that our daughter is divorced and we have to refer to them as her ex-husband's father/mother etc. It's easier when talking to our grandsons as we can refer to them as Grandpa and Nana.