He put on ihraam then he decided not to do Hajj

If he puts on the izaar (lower garment) and rida’ (upper garment), but he does not have the intention of doing Hajj or ‘Umrah, and he does not recite the Talbiyah, then he has the choice. If he wishes he may start Hajj or ‘Umrah, and if he wishes he may not do so. There is no sin on him if he has already done Hajj and ‘Umrah, but if he intended to start Hajj or ‘Umrah then he cannot cancel that and change his mind, rather he must complete that for which he entered ihraam, in the manner prescribed in Islam, because Allaah says (interpretation of the meaning): 

“And perform properly (i.e. all the ceremonies according to the ways of Prophet Muhammad), the Hajj and ‘Umrah (i.e. the pilgrimage to Makkah) for Allaah”

[al-Baqarah 2:196]

Hence it should be clear to you that once the Muslim starts Hajj or ‘Umrah with the appropriate intention, he does not have the right to stop that, rather he must complete what he has started, because of the aayah quoted above, unless he has made a condition in case something prevented him from doing it. In that case he may exit ihraam, because when Dabaa’ah bint al-Zubayr said to the Prophet (peace and blessings of Allaah be upon him), “O Messenger of Allaah, I want to do Hajj but I am not sure (I will be able to do it?),” he said, “Go for Hajj but make the condition that you will exit ihraam if you are prevented from doing it.” (Saheeh – agreed upon). 

And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad.