Even though he was in the proceedure of parking and was in reverse, he should NOT have taken it for granted that there was no obstruction or vehicle or even people passing, he should have checked before moving off.
He is clearly to blame for this and I hope you got all his details to pass on to your insurers.
Report his comments said to you at the time and make notes on everything e.g. vissibility, weather, night/day, busy/quiet carpark.
Don't let your insurers pay this guy as your no claims will rocket.
Take him to court if you have to.
Good luck!