بسم الله الرحمن الرحيم
Since you named him after the famous Sahabi Musab Ibn Umair (may Allah be pleased with him) but you failed to understand correct pronunciation, however, when now you have known the correct pronunciation then you should keep the name as Musab. The word مصبwithout عmay not be used as name.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.