Minecraft – Run specific commands without op on a minecrft server

One thing you can do is type in console is "whitelist add username"

If you can not do that,

http://dev.bukkit.org/bukkit-plugins/permissionsex/

Create a group,

  Admin:
    default: false
    build: true
    permissions:
    - '*'

If you create a group set it to false If you want only certain people to have it or true if anyone can have it.

In console type "pex user add [USERNAME] group add Admin"

If you do not have that try /reload

Most likely it's not the client, but the area he is building in. If something about his structure causes high server load (such as a large, complicated redstone circuit), it will only lag the server when that chunk is loaded. If he's building somewhere far away from spawn, his chunk will only be loaded when he is online, because no one else can find it.

You can easily test this theory by teleporting to his logout location and seeing if the lag starts up again.