As far as I know, there is nothing wrong with saying this du’aa’. If he makes du’aa’ saying other words, such as saying, “O Allaah, bless me with good offspring, O Allaah, grant me righteous offspring,” and similar du’aa’s, all of that is good. And he could recite the aayah (interpretation of the meaning): “O my Lord! Grant me from You, a good offspring. You are indeed the All-Hearer of invocation” [Aal ‘Imraan 3:38].
And it is recommended for him to go and see specialist doctors, because he may not be infertile as such, but there may be a problem that can be treated. And it is suggested that he should marry a woman who has previously given birth, because the problem may lie with the wife he has, not with him, and Allaah may bless him with good children in this way. It was narrated that the Prophet (peace and blessings of Allaah be upon him) said: “Marry the one who is fertile and loving, for I will be proud of your great number before the nations on the Day of Resurrection.”
And Allaah is the Source of Strength.