As a practice nurse ( and mother), I agree with the many posts saying tell them, on the day is fine, before, not so much, it gives them too much time to ruminate. Also agree, age appropriate, ( and you know your own child best), bright and breezy with treat afterwards.
One word of caution, if giving a sweet or chocolate button, don't (!) put it in their mouth as the 'prickle' goes in. They will most likely inhale sharply as its administered, and could choke on the sweet.
This happened to my colleague and she didn't see the parent pop it in at that direct moment. The child was fine, but needed sharp administration of hand between the shoulder blades to dislodge the sweet. It was a distressing event and I've never forgotton her telling me.
All the best, OP. Practice nurses have done thousands of these jabs and you would be very unlucky to find an unskilled nurse; it's usually done efficiently and swiftly.
Do remember to take his 'red' book, or whatever colour it is in your area. It's amazing how long into the future one needs to look back at vaccination records for forms, etc...