If he is throwing it on the air and bracing then yes, his back will hollow. You however, want positive tension. Because he doesn't have the strength or balance to carry himself in a 'round' outline at this early stage, something has to give. Often when you start to rev up the engine, the horse will feel most comfortable with the head and neck raised, as opposed to braced. This will lessen as he finds balance and builds muscle with active hind legs.
You need to ensure that you keep a steady, elastic contact with the bit, not pulling back of side-to-side. Once he can motor off on that hind end, you can start looming at adding some leg yield on a circle and other exercises to develop lateral suppleness. THEN he will start to 'come onto the bit'.
As said neurosis times above, this is very difficult to achieve without help on the ground, someone telling you in real time when to give or take aids. That is why a good coach is invaluable. Unless you have felt it before, or have someone telling you 'yes, now' on the ground do that you can store that feel in your memory bank, you will find it very difficult to feel when the work is correct, as a beginner.
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