Absentmindedly he put his Ihram on his head for a short while
a Muhrim (pilgrim in the ritual state for Hajj and `Umrah) forgetfully does any of the things which are unlawful to do during Ihram, it is obligatory to desist from doing that thing as soon as they remember. There is no sin on you and you do not have to make a Kaffarah (expiation), for the Saying of Allah (Exalted be He):
Our Lord! Punish us not if we forget or fall into error
Allah (Exalted be He) granted that and said:
I did (pardon you).
It is authentically reported that the Prophet (peace be upon him) said:
My Ummah (nation based on creed) has been pardoned for what they do by mistake, forgetfully, or under duress.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.