This is a great question. In almost every part of the novella, George is like a father to Lennie. Let me give you two examples.
First, in the beginning of the novella, George and Lennie are by a body of water resting. When Lennie drinks deeply of the water, George chides him. He does this, because the water is not clean. Lennie will get sick. Here is what the text says:
“Lennie!” he said sharply. “Lennie, for God’ sakes don’t drink so much." Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. “Lennie. You gonna be sick like you was last night.”
This quotes shows that George is looking out for Lennie, as a father would do.
Second, when Slim comments to George that Lennie is an incredible worker, George smiles. This smile shows that George is proud of Lennie's strength and his ability to work. This point, too, shows his paternal side. Here is what the text says:
George spoke proudly. “Jus’ tell Lennie what to do an’ he’ll do it if it don’t take no figuring. He can’t think of nothing to do himself, but he sure can take orders.”