This question isn't necessarily easy to answer because the narrator doesn't expressly tell readers how Jim reacts to Della's new hairstyle. The text does tell us a lot about how Jim doesn't react.
His eyes were fixed upon Della, and there was an expression in them that she could not read, and it terrified her. It was not anger, nor surprise, nor disapproval, nor horror, nor any of the sentiments that she had been prepared for. He simply stared at her fixedly with that peculiar expression on his face.
The narrator lists a lot of things that Jim's expression was not, but there are still plenty of expressions that the "peculiar expression" could be. I like to think that his expression is a nice mixture of humor, acceptance, and irony. Jim doesn't get mad at his wife, and he tells her that he likes her no matter what her hair looks like.
"Don't make any mistake, Dell," he said, "about me. I don't think there's anything in the way of a haircut or a shave or a shampoo that could make me like my girl any less."
Jim does know what he bought Della for Christmas; therefore, he immediately knows that he is holding a gift that can't be immediately used by his wife to beautify her hair. It is the last thing that he expected to see when he got. Jim probably immediately sees the irony in the situation, and he has to check a couple of times to make sure that he is seeing things correctly.
"You say your hair is gone?" he said, with an air almost of idiocy.
Jim's peculiar expression is a perfect way to describe his reaction because it is so vague. It lets readers imagine exactly what his expression is that even his wife can't figure out. We are given enough detail to know that his expression isn't negative, and then the narrator strings us along until we finally understand why his expression was so unique.
They were expensive combs, she knew, and her heart had simply craved and yearned over them without the least hope of possession. And now, they were hers, but the tresses that should have adorned the coveted adornments were gone.