If a pilgrim fails to throw the pebbles, his Hajj is valid but he has to offer a sacrificial animal.
In this regard, the eminent Muslim scholar Sheikh Yusuf Al-Qaradawi states: “According to the majority of scholars, throwing the pebbles is wajib (requisite), not fard (obligatory). Anyone failing to throw the pebbles is required to offer a sacrifice, as reported by Ahmad, Muslim, and An-Nasa’i from Jabir, who said, “I saw the Prophet (peace and blessings be upon him) riding his mount and throwing pebbles on the Day of Nahr (Dhul-Hijjah 10), and saying, ‘Take your rituals from me. I do not know whether I will be able to perform another Hajj after this one.’
Based on this, your Hajj is valid but you have to offer a sacrificial animal. If there is crowding, the pregnant and the old and weak people can authorize someone to throw the pebbles on their behalf. Jabir said, “We performed Hajj with the Prophet (peace and blessings be upon him) and we had some women and children with us. We (adults) uttered Talbiyah and threw pebbles on behalf of the children” (Ibn Majah).”