بسم الله الرحمن الرحيم
(Fatwa: 1128/940/H=09/1440)
If after full efforts he is unable to pronounce correctly then too his salah will be valid. In future too he should try his level best and do not do any negligence in this regard.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
