Assalamu alaikum wa rehmatAllah,
If he is unable to change it, his prayer is valid.
I have two local masjid, one is ahle hadith and the other deobandi. At the ahle hadith one the imam has recieved naseehah regarding changing the meaning when leading fatiha he says siratala zeeena replacing the thaal with a zaa. Yet he has failed to make the slight change. Islamqa states the salah is not valid. So at which mosque should I pray the congregation. Jazakallahu Khairan