Jumping “hard” seems to have many different meanings/nuances.
So, is he overjumping? Or landing too heavily? Cracking his back too much over fences? Or is he rough to sit over fences, the type that launches you out of the tack hard? Rushing hard?
From your description, I am guessing it is just overjumping and not jumping from a good distance.
Overjumping is an easy fix - almost all green horses overjump for a while - until they get 100% comfortable going over fences. It just takes time and patience. My favorite exercise for horses who overjump is to just jump the same two verticals, over and over: put them across the diagonal, pick up your canter, jump over it and change direction, jump over the other one (also on diagonal) - endless figure 8 loops: after the third or fourth pass they will start to settle and not throw as much effort into jumping it.
This will help train your eye and his, while delivering consistency - the jumps won’t change, the pattern doesn’t change - it allows them to settle into a pattern while you work on delivering a soft, amenable approach: since he is landing almost hind first after the fence my bet is that the approach is messy, rushed, or there is fighting: they’ll land hind feet first when they are unbalanced in the take off.
Part of it is them developing their style: some horses will just jump hard as in be hard to sit over fences; others are much softer - some will always land heavily, others will always crack their back. It’s important to inspect what you are doing moments leading up to the fence: green horses IME will definitely overjump worse if their rider is bracing, pitching forward, or fighting them to the fence.
So I would be working on balance and rhythm leading up to the fences. I set my verticals at 2’3". That’s big enough for a green horse to respect, but not so big that it requires tremendous effort to go over. Keeping the fence a little smaller will also help in terms of ‘going longer’ - the smaller the jump, the less effort required to go over it, the longer you can work on perfecting it in one sitting.