My son wants to marry a girl from the Bohra sect

In the Name of Allah, the Most Gracious, the Most Merciful.  

As-salāmu 'alaykum wa-rahmatullāhi wa-barakātuh.  

The Bohras are a sect within the Ismaili Shiite branch of Islam. Unlike the Twelver Shias who follows 12 Imams, Bohras believe in 21 Imams. Bohras also have many divisions within themselves like Dawoodi Bohra, Alavi Bohra, Sulaimani Bohra, Progressive Bohra, Qutbi Bohra but the Majority of them are Dawoodi Bohras. 

The Dawoodi Bohras are Shias of the Mustaalian division of the great Ismaili sect. The main differences between their beliefs and practices and those of regular Muslims are: the Dawoodi Bohras pay special attention to Ali (Radhilallahu Anhu), to his sons, Hassan (Radhilallahu Anhu) and Hussain (Radhilallahu Anhu), and to their high priest, the Mullah Sahib of Surat; they pay special attention to circumcision; they reject the validity of the three caliphs, Abu Bakr Siddiq (Radhilallahu Anhu), Umar (Radhilallahu Anhu), and Uthman (Radhilallahu Anhu).  

Many beliefs of Bohra Shias are contrary to the Quran and Hadith and are therefore regarded as Non-Muslims.  

If what you mean by a Bohri girl is someone who ascribes herself to the aforementioned beliefs, then it will not be permissible to marry such a girl.  

We advise you to kindly explain to your son that it is not acceptable to marry such a girl.  [i] 

And Allah Ta'āla Knows Best 

Checked and Approved by, 
Mufti Ebrahim Desai.