Are both owners (joint owners) of Flat are required for registration?

Hi Sir, The flat that I intend to purchase is owned by 2 persons - a joint property (son and mother). First name is in his name and Second name is in his mother's name. Son lives in Dubai and Mother lives in India. Both Son and mother are willing to sell the property. However, Son is exploring if he could manage without him being physically present at the registration office and avoid travel all the way from Dubai. So i have 3 questions in this case with respect to flat registration in my name. 1. Is it must that both son and mother are required at the time of flat registration? 2. Is there any legal option that registration is still executed without his Son, and that the registration is legally valid in India? What is the best legal option if there is any and how much does it cost? Would it be cheaper compared to his travel cost to India? 3. Is it safe for the purchaser, that the Son in future could create any legal claim on the property? Thank you. Appreciate your quick guidance.