You could practice on teddies first at home - talk about special medicine to make teddy big and strong, might hurt but it's very very quick - and then give teddy a nice treat for being brave?!
If you're really worried ring the surgery and ask to speak to the nurse (who'll prob be doing it?) and ask for some advice - that way she'll also know before hand that he (and you!) are nervous.
A good choccie treat and taking teddy in for moral support would be the thing I'd suggest - as well as talking about what you're going to do afterwards - going to the shop, going home to bake some biscuits, going to the park... something you can natter about for aaages in the waiting room and at the time and afterwards to change the subject - and then follow up on it obviously (bit harsh not to
)
Really hope it goes ok xx