Gregor's father wears his work uniform even after he returns home from work to symbolize how is doing his part to support the family.  His wearing the uniform is an arrogant statement to the family about how important he is.  It would also be a logical inference that he wears the uniform so that Gregor can see him in it and feel even more bad about his metamorphosis.  Gregor was the the provider for the whole family, and now they all must go out and work for a living.  Gregor was proud of his ability to take care of the family, but it is subtly revealed that the family are all capable of working and that Gregor didn't have to carry the burden of the family debts and expenses by himself.  The father had a substantial amount of money left over from his business and the family is also able to save a portion of the money Gregor gives them.  This money could have been used to get the family out of debt sooner and allow Gregor to get on with his life sooner, but the family took advantage of Gregor and thoughtlessly let him continue working for a company and a boss that made him lonely and miserable.  The father is angry that his comfortable retirement and living off of his son is over, and he is asserting his anger in this rather passive-aggreessive manner.

The scrupluousness with which he holds himself, as described in the passage above is further evidence of the pride and arrogance of Gregor's father.  He is not a weak, frail man incapable of working; he is quite fit and quite capable.