Bismillah

Al-jawab billahi at-taufeeq (the answer with Allah's guidance)


1) To perform Salah behind Shi’as, the Salah will not be valid. (Aap ke Masaail Aur Unka Hal vol. 2 Pg.239, see also: Ahsanul Fataawa)


2) Let him attend the prayers etc, as long as he doesn't do anything against the rules and ettiquettes of the Masjid. However, you must try to make effort in Dawah, inviting him to the teachings of the Ahlus Sunnah Wal Jamaa. Ensure this is done with gentleness, calmness and wisdom to avoid Fitna.


And Only Allah Ta'ala Knows Best.