The diffuser covers a large area (up to 650 sq. feet) so it doesn't need to be right where the cat is. I would suggest putting it in an open area so that it can diffuse, ie. don't put it in a plug that is behind furniture so that the vapour settles on the furniture.
Spraying the room would definitely help, especially around the door.
If he's attacking you, you might want to try some Rescue Remedy in his water to help calm him down.