1)
What pains his flesh?
1.
Rashi: Worms pain a Mes like a needle pains live flesh (Berachos 18b).
2.
Ibn Ezra: "Yich'av" is melting, decaying and destruction, like "v'Chol ha'Chelkah ha'Tovah Tach'ivu ba'Avanim."
3.
Malbim: It is not his children's pain - he does not know of it! Rather, it is his own pain.
2)
What do we learn from "v'Nafsho Alav Te'eval"?
1.
Malbim: Also his Nefesh's mourning is for itself, for he does not know about his children.
