Signing another's name can put you on shaky legal ground. Without proper legal documentation, such as a power of attorney, signing your father's name is forgery and a clear violation of the father's credit card agreement. However, if the father has given permission and willing to pay the charge on the card, the son may be able to sign. The best situation is for the father to provide cash or accompany his son for the purchase instead.
When Problems Arise
The key is that the person signing the credit card purchase needs to have clear permission from the cardholder. If there is a disagreement later over whether the signer had permission, he may be liable for the purchase and may be legally vulnerable to fraud or forgery charges. Keep in mind stores may ask for identification in order to prove that it's his credit card so, without dad there, he may not be able to complete a purchase with the card. Again, signing another person's name for a credit card purchase can constitute illegal forgery.
