Why did Avraham pray to Hashem for Yishmael? what was the reason ?

ב"ה Shalom My assumption is that you are referring to Bereshit 17:18 where Avraham prays "May it be granted that Yishmael lives before you!". A simple glance at Rashi would tell you that Avraham in his modesty, after being promised a son from Sarah, was simply saying to Hashem that he is not worthy of such a Heavenly gift and he will suffice that Yishmael live and serve Hashem. All the best