Grumpy Horse in stable, how do I approach him without him bitting me?

At my stables there is a horse who is absolutely lovely to ride but when ever I go in to groom him or tack him up his ears go back and he pulls faces at everyone. He is telling me to back off but I need to go and groom him. Any ways on how I could enter his stable safely and not get bitten? Or even how to stop him from getting mad?