I'm awfully sorry for the poor guy. I'd like to make a recommendation however.
If you're going to be giving him the syringe, let him see you with it. Don't hide it behind you. If he's going to develop a fear of you because of this, make it be related to the syringe, not to YOU in general. If it's not time for the syringe, don't let him see you with it.
In his mind, make him think:
You + syringe = bad
You + anything else = good
This will focus his fear on the temporary thing, not you.