It can be done

My mother in law taught me how to medicate cats. She raised silver perisans professionally for years.

Mother cats carry their kittens by the nape (back) of the neck. Pick up your cat like the mother does but keep his back feet on the floor. Straighten him up until his is fully veritcal--keep your hands behind the cat. Keep pulling him back the further the better, don't jerk or pull, move slowly and you are completely in control of him. His mouth will open and you can insert the medication. I used this method one day at the vets office and he had never seen it before.