I prn
Severin the same way you do, but I can't say I like it much. It calls to mind words like "sever" and "severe". "Sevrin'" is how I would prn "severing", so it's a no-go for me. I guess you could call him Sev, though, which wouldn't be too bad.
As for Severn...I think I prefer it to
Severin, but only if it's prn seh-VERN. Masks the "sever" part that way.