Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-Uthaymeen, may Allaah preserve him, who replied as follows:
There is nothing wrong with leaving instructions in one's will that if this son does not return to Islam before the father dies, he will have no share of the inheritance.
Can he write down specific shares with the lawyer?
He should write it down according to Islamic law, because he does not know, some of the heirs may die before him.
So he can write in his will, kept with the lawyer, that his estate should be shared out according to Islamic law, and that if this son is not praying and has not returned to Islam before the father dies, he will not have any share of the inheritance.
And Allaah knows best.