For an adult dog that was not trained to accept nail trimming as a puppy, I would recommend a muzzle. It is understandable that he is apprehensive, but he also must learn that he must tolerate the procedure, and threatening to bite (or actually biting) is not acceptable behavior.
One thing you might try is a nail grinder instead of clippers, some dogs are more comfortable with grinding, since the snap of the clippers can startle them. Another tip is to trim nails often, at leat twice a month and maybe even weekly. The more routine the procedure becomes, the less he should react.
You also need to have a calm, assertive attitude when it is time to trim nails...if YOU are nervous and apprehensive about how he will react, he is going to pick right up on that, and he will in turn feel more stressed.
Puppies here (I am a breeder) learn early on that you lay upside down in owner's lap, owner snips nails, a belly rub and a treat are given, and we are all done! Takes about a minute per dog.