The Prestige V + WNV with Halvogen still says WNV.
Here is a sheet about WNV. Basically, if he's had it, then just give it once but if not, he will need a boost in 4 to 6 weeks the first time. If you are unclear about his vaccination history, it is better to do the boost in 4 to 6 weeks to be SURE. You do NOT want your horse to get WNV! West Nile Virus - AAEP
If you want to do the booster yourself, a good video on Youtube about it is:
"How to give a shot in the neck" by Padmavideo.
It shows Dr. Thomas Casselberry DVM giving an instruction on vaccinating your horse in the neck.
I would link it, but my browser won't let me link videos right now for some stupid reason.