I would suggest that you try putting one drop of Grapefruit-Seed-Extract in a small amount of really smelly canned cat food and mixing it well. You may even want to warm up the cat food a little. The easiest way is to run hot water on the unopened can for a few minutes.
Do not put it straight into his mouth. Undiluted Grapefruit-Seed-Extract can burn mucous membranes, which include mouth and stomach. Putting four drops into a capsule and forcing that down him does not sound like a good idea to me at all. When the capsule would dissolve in his stomach it could burn his stomach mucous membranes.