If someone holds like this, they cant get away from you:
Except do this on the floor, and put a knee on top to help hold down. make sure the persons forearm is across the dogs neck and the legs ar stretched out. If they cant get their head up or their legs under themselves, they cant go anywhere. Covering the dogs eyes with a towel helps too, and putting peanut butter in the mouth (or something they really really like) helps.
Before resorting to force though, I would try some desensitization:
1. pick a food the dog really likes, and will only get when he gets his nails trimmed.
2. start keeping the trimmers out and give him the treat if he stays calm
3. move up to putting the clippers near him, give him a treat if hes calm
4. the dog must stay calm, if he gets nervous, go back a step and work with him until hes calm, then give him a treat
5. step by step get closer to the act of trimming his nails: next would be someone holding him for you, you with the trimmers in your hand, then add touching his foot with the timmers, then add trimming one nail in the next step, then one foot, then a few feet, etc
pretty soon, nail trims will = awesome yummy treat time!!
spread it out into short sessions once or twice a day, not all at once (one day do the first step, the next day the next step, etc.)
make nail trims a part of everyday life
if he ever gets scared at all at any point you MUST go back a step and work on it until hes calm.
It takes time, but paying for nail trims adds up
have cornstarch ready in case you cut a nail too short