More than likely, you will have to give him a shot. The last time I went to the vet for this, they gave me six doses already measured out in the needles... I kept them in the fridge, and injected once every few days (can't remember what the prescribed length is).

If they find out that your snake does have an RI---or want to treat just in case---ask them to demonstrate how to inject. It's incredibly easy, but intimidating if you've never seen it done.