You can OP a player by typing into the console "/op (username)"
If you wish to create a rank for him, then you can install Essentials, LuckPerms and EssentialsChat.
To create a rank, type in /lp creategroup Admin then to set the prefix do /lp group Admin meta setprefix "(prefix)"
To give him the Admin rank type /lp user (username) parent set Admin
More can be found at https://github.com/lucko/LuckPerms/wiki/Command-Usage
If you have any other questions, feel free to DM me on discord (Criticyl#0869) or reply to this response.