Try to find a tube that is the right size to slide his head and neck into, like the tube at the end of a roll of paper towels might be the right size for you. Also work on a smooth surface and don’t allow him to get his tail wrapped around anything, like the end of the table top. If he can get traction/leverage it will make giving the injection very difficult. He will still be able to jerk but not near as much.
Also wear old clothes as he may decide to musk you.

Good luck with the injections. I hope this helps.