Adult supervision needed.
Any one who holds the stick forward on a PA28 take off roll should not be let out without a grownup to look after him, those above have well detailed the reasons.
It sounds to me as if your former instructor is a wannabe airline pilot, forward pressure on the stick up to 80 KTS is a Boeing 737 thing to keep nose wheel steering authority untill the rudder gets some airspeed.
I would guess I am looking at about 1000 hours of DR400 time and the best way to get the aircraft off the ground on grass is to hold he stick full aft at the start of the take off run and as the airspeed increases past around 30 KTS smoothly move the stick forward to a position just aft of neutral, then at about 50 KTS move START a smooth rotation.
This assures minimum load on the nose gear and moving the stick forward after 30 KTS keeps the same nose gear protection due to the increasing effectiveness of the Stab due to increased airspeed but reduces drag and the risk of getting the aircraft into the air below the normal stalling speed due to ground effect.
Enjoy the DR400, it is without doubt the best aircraft in its class.