It is not unusual to find Arabic using an initial ayin in words borrowed from Aramaic and the dropping of final ayin is evidenced by the form Yisho of the Manichaean “köktürkish” fragments from Turfan (footnote 'so sometimes in the Iranian and Soghdian Manichaean fragments, see Henning, Manichaica, ii, 70, and Manichaisches Beichtbuch) [Arthur Jeffery, Foreign Terminology in the Quran p. 220]
Not so sure that it is simply a matter of 'dropping' the final ayin or that Ishu goes back to a tradition about a man named Jesus. As I have noted many times here, the same word in Hebrew means 'His (= God's) Man' a figure that appears as early as Enochic literature, manifests itself as a title of Moses, makes its way into the earliest Qumran and Samaritan material and ultimately heretical Christianity.
