You must first be fully informed yourselves, to be able to inform him. The various types of genital HPVs (that cause wart-like lesions) may remain in our body for weeks, months or even many years after the initial infection.
If your partner happens to be your first sexual partner, it is almost certain that he transmitted the infection to you. In most cases however men do not know that they have been infected by HPV. Only men who have developed genital warts in the past know about it. The others are unaware, since they usually have no symptoms or visible lesions. Many men may suspect that they have HPV because it was diagnosed in previous partners (if they were told about it).