tell him it's like a hard twisty pinch, ask him if he knows what that feels like(chances are another child might of done this to him already), and that when a pinch happens, it hurts then it is over,
then offer to let him pinch your arm (or daddy's
), let him do it, then say ouch that hurts, lets rub it better then ask if he wants you to do one on him, if he does do a little one, then talk about how to make it feel better,
then talk about how we never hurt a person on purpose, but sometimes we have to do a little hurt (injection) so we avoid much bigger illness that wouldn't get better with a rub,
a friend who has a lot of children, introduced me to this method, I was very sceptical but children seem to be interested in discussing it like this, and it worked really well, part of distress is the surprise that something painful is happing suddenly without prior discussion. Mine all having had the chat, went on to take it really well, no real fuss.
If he knows it going to be like a twisty pinch then he can prepare himself, and he knows it will feel better quite quickly, and he can go and choose his treat for being brave.