Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
As to what follows:
With regards to answering your question we say, success is by Allah Almighty.
What is obligatory for him according to the view of many scholars is to make 4 rak’ats Isha, in this case it is not permissible for him to do qasr (praying Isha 2 rak’ats) because he is following an imam who is performing swalah with more rak’ats than his, so if he joins the Imam then it is obligatory to complete 4 rak’ats.
Ibn Abas was asked regarding praying swalah with complete rak’ats in Makkah and shortening in Mina and he said: it is sunnah to do that.
Some scholars are of the view that it is permissible to do qasr in this case whether joined the Swalah from its beginning or he got less than 2 rak’ats or less than that. What is correct is that it is not permissible