Well actually I am 99% sure he was a captain he has a captain robe!!!
And I'm 70% sure he was a captain or royal guard, hell yeah!
. And don't tell me that it doesn't say anything! Only captians have these robes, not even vice-captains. I don't think it's different in royal guard. I don't think he was part of central 46. Cos 1 we don't really now about their clothing,and second I don't think that central 46 are the one who can fight. They are just trusted faces of royal family. BUT Kubo showed as the robe for some reason!
Isshin's captain robe