high iso setting or using night vision.
I see on the link you posted that he does workshops, so it may be a good idea to attend one of these, especially as you aspire to his style of photography!
Not personally a style I'd copy. I wouldn't feel comfortable hitting a seal pups night-adjusted eyes with a burst of unexpected flash, or even a torch.This is the type of style Id love to practice with if it helps http://jamiehallphotography.photoshelter.com/image/I00006G1Fa3jlERE
If you look at the photo it's very obvious that there's an artificial light source involved. He even identifies it as an Elinchrom Quadra Ranger in the tags.With such a long exposure I am not sure if he used any external lighting...