Kirk,
He's flying a NexStar, which is a tricycle gear trainer.

Holding up elevator at the beginning of the take off roll with a trike gear plane on grass is usually done to get the weight off the nosegear. Otherwise it tends to drag too much in the grass preventing the plane from getting up to take off speed.

Still not too sure of the problem though. Are you holding up elevator too long, not long enough? I've flown some student's trainers that required me to hold full up until the plane came off the ground (which is too soon for full flight), then had to push in a lot of down for a second to keep the plane a couple feet off the ground until good flying speed was reached.

If your's is like that, not enough power in grass that is too long, then it WILL be difficult to get use to taking off.
Dennis-