Legal analysis: if the son is entrusted by his father or ratified by his father afterwards, it is legal and effective according to the provisions of civil law for the son to sign as the client. However, the son signed his father's name, which is an act of fraudulent use of his name. The signature is invalid, and the son is responsible for the consequences.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 144 A civil juristic act performed by a person without capacity for civil conduct is invalid.

Article 146 A civil juristic act by the actor and the counterpart with false intention is invalid. The validity of a concealed civil juristic act with false intention shall be handled in accordance with relevant laws and regulations.

Article 153 A civil juristic act that violates the mandatory provisions of laws and administrative regulations is invalid. However, unless mandatory provisions do not invalidate civil legal acts. A civil legal act that violates public order and good customs is invalid.

Article 154 A civil legal act in which the actor colludes with the counterpart in bad faith and damages the legitimate rights and interests of others is invalid.

Article 505 The validity of a contract concluded by the parties beyond the scope of business shall be determined in accordance with Section 3 of Chapter VI of Part I of this Law and the relevant provisions of this Chapter, and the contract shall not be deemed invalid because it exceeds the scope of business.