It's important to get technique right first. Try getting the tackler to tackle by first starting on his knees. Get another boy to run at him and he makes the tackle , head up, gripping his hands around the attacking boys legs then rolling with the momentum with his head beside the attackers hips / thighs in the correct position. Once this is mastered move to one knee , then move to being on both feet. Boys need to realize it doesn't hurt making a tackle and good technique will help prove this to them. Only then will they develop enough courage to try to tackle And tackle correctly.