Hannah does this often – I push her off or say “hey!” as soon as she starts digging because I know the nip is coming. You could try a high pitched yelp if the “hey!” isn’t effective on him. If he won’t stop, just push him off and turn your back to ignore him. He should get the idea that it’s not ok. Hannah still does it, but she usually just digs for a second and then stops to look at me while before she would dig and dig and then nip.
Baby bunnies are often pretty nippy – they’re exploring their world the best way they know how, but it’s usually not too hard to teach them it’s inappropriate to nip humans. When Otto was a baby he would nip at my jeans, especially my knees if I was sitting cross legged. Yelping didn’t work for him, so I just left his area (I’d sit right outside his pen) with my back turned. After 30s or so I’d go back in. He learned pretty quickly that his playmate left when he started nipping and he’s never nipped again as an adult.