OK. The best advice for you is to try and stick your bum out as far as possible, and raise your A-hole to facing horizontal, as if you are inviting anal or something…
It is weird at first, but if you get over my horrible analogy you will have much better form.
Also, clench your scapula together as if you are doing a row without the arms.
Thirdly raise your back and hips a lot sooner. You may scrape your shins with the bar at first, but then you will get more flexible over time and it will stop.
As everyone else said, drive through your the balls of your heels as if you are going to win a tug-rope war.
That’s all solid advice, but none of it is going to really help him fix the imbalance he has. His left side is not able to hold the negative as strongly as the right. It’s POSSIBLE that it’s an imbalance between the left and right side obliques/erectors, or even his traps.
A pretty cool movement a friend of mine showed me is to dead lift using dumbbells. The weights, though, are uneven. So for example…if you can pull an easy 200, you might want to use a 90 and a 60.
Pull for 4-6 reps, then switch. I’ve done them for a few weeks now and you can really feel your abs and obliques firing to keep you straightened out.
Obviously maintaining good form is important here, or kiss your back goodbye.