What physical change did Gregor notice in his father?

From the text:

He was standing up straight enough now; dressed in a smart blue uniform with gold buttons, the sort worn by the employees at the banking institute; above the high, stiff collar of the coat his strong double-chin emerged; under the bushy eyebrows, his piercing, dark eyes looked out fresh and alert; his normally unkempt white hair was combed down painfully close to his scalp. He took his cap, with its gold monogram from, probably, some bank, and threw it in an arc right across the room onto the sofa, put his hands in his trouser pockets, pushing back the bottom of his long uniform coat, and, with look of determination, walked towards Gregor.