Children will sometimes make a stand about all sorts of things, it is unfortunate that yours has chosen, albeit subconsciously, to make a stand on injections.
It is now a battle of wills, and one you must not lose. At 7.5 years you should explain, quietly but firmly, that the injection needs to take place and that you would rather do it with consent, but if not, because it is so important, you will have to do it anyway.
Rewards are good, but also consider taking something away for lack of co-operation, something small and simple is OK, it is establishing a principle, co-operation gets reward (don't forget the praise) non co-operation gets disapproval and a mild punishment.
It is, from what you say, a mild form of attention seeking and really needs to ne nipped in the bud, it's not as if you have any choice, it's just got to be done.
I should mention that I have no experience of injecting children, but plenty of experience in helping them to conform, it's not easy so good luck!
H