[Gen 37:35]:

"And all his sons and all his daughters rose up to comfort him; but he refused to be comforted; and he said, For I will go down into the grave [=sheol] unto my son mourning. Thus his father wept for him."

The Hebrew points to Jacob's refusal to be comforted by stating:

[Heb]: said - down - sheol - son - mourning = Jacob is saying that he will mourn his son until he goes down into Sheol TO HIS SON.

This part of the verse could not be interpreted as Jacob mourning FOR HIS SONuntil he goes to the grave but rather it is to be interpreted as Jacob mourning [for his son being already implied] until he goes down to Sheol [afterlife location]TO HIS SON[in Sheol].

Since the subject of the passage is Jacob being told of Joseph's demise at the hands of carnivorous animals it would be redundant to restate for whom Jacob would be mourning. So a rendering '[Joseph said] I will go down to Sheol mourning FOR MY SON.' is incorrect rather than '[Joseph said] I will be mourning until [I] go down into SheolTO MY SON' which is correct.

Furthermore, since Joseph was eaten and there would therefore be no 'grave' for him, and since one is not buried in a common grave with relatives and since the Hebrew word for grave, (kever), is not used here, sheol is; then grave must be ruled out. So it must be that Jacob is saying that he will be in mourning all this life [for Joseph] until he goes to Sheol to [be reunited then with] his son who is there now.

Also, considering the Hebrew word "yarad [Str # 03381]" = "down" = one does not go down into the grave since graves were normally up: in tombs or rock coverd graves above the ground because of local carnivorous animals. On the other hand one does go down into the nether world of the after death existence = sheol.

Jamieson, Fausset and Brown's Commentary

"Genesis Chapter 37


and he said, For I will go down into the grave unto my son--not the earth, for Joseph was supposed to be torn in pieces, but the unknown place--the place of departed souls, where Jacob expected at death to meet his beloved son."