Technique is much debated. One thing in the forward stride is that the upper body must lead where the lower body is going. You shouldn't be "swaying" the body, instead you need to be utilizing all the power on one side to drive/push forward. This sounds overly simply but is very much true. Consult Robbie Glantz or Sean Skinner's videos as proof.
but he told me when I sway it forces me to put all my weight onto one stride, which will give me a bigger stride..... but what happens when I make a habit of this, and cant make my head straight
Is he having you do this as a temporary thing, or permanent? Sometimes making someone skate a certain way may seem silly, but the intent is to counteract some problem in their stride - kind of like tying your lower hand onto the stick if you have a tendency to skate around with only one hand on your stick all the time. It's not meant to be forever, but it can break the habit. If you're not loading up your leg on each stride, maybe he's trying to make you exaggerate the leg-loading by swaying.
Without seeing your stride, it's kind of hard to answer...