Need help giving frito shots!!

Our Boston (Frito) has been in an allergy study now for over 2 years. They finally narrowed it down to what he is allergic too, and now we have to give him a shot every three days for a year! He was fine the first couple of times but now he is scared to take them. The vet showed us how to do it, we grab the extra skin on the back of his neck and pull it up, then we insert the needle and push the plunger down. We have to aim it towards his head so he wont run backwards and jam it into his head further. But ever since last week he has been jerking his head as soon as we stick the needle in. Its very stressful for us because its something we just cant not do. It has to be done. Does anybody have any advice for us? Thanks a lot in advance!