Or HaChaim on Genesis 48:19/אור החיים על בראשית מ״ח:י״ט

1 וימאן אביו ויאמר, His father refused, saying, etc. The reason the Torah uses the word "he refused," is to indicate that Joseph had taken hold of his father's hand to try and place it on Menashe's head.

2The reason Jacob repeated: "I know my son, I know, etc." is that Jacob wanted to answer both of Joseph's doubts as we have already explained. To Joseph's remark "not so my father," Jacob replied that he was well aware that Joseph had already placed the children in such a position that he, Jacob, did not have to cross his hands in order to put his right hand on the head of Menashe. By adding the word בני, my son, Jacob hinted that he had given Joseph credit for placing the boys in the position which he considered as appropriate. Concerning Joseph's specific concern that the firstborn be accorded the privileges due him, Jacob also said: "I know;" he assured Joseph that Menashe too would attain greatness. Joseph should not think that Jacob considered his firstborn as unworthy or inferior. If he had nonetheless decided to place his right hand on the head of Ephrayim this was in view of Ephrayim's descendants outranking Menashe's descendants at some time in the future. Ephrayim was destined to become מלא הגוים, meaning that at that time he would occupy a higher position though at the moment Menashe fulfilled his role of the firstborn perfectly. Tanchuma elaborates that Joshua would be descended from Ephrayim whereas Gideon would be descended from Menashe, both leaders of the Jewish people during their respective lifetimes. Both had already been destined for their respective roles in life; Jacob only gave recognition to this by the manner in which he placed his hands when blessing both these children.

3Alternatively, Jacob meant that he was thoroughly familiar with Joseph's arguments; when Joseph had hinted that the birthright should determine on whose hand Jacob was to place his right hand, Joseph was wrong, it was not the order of birth which determined matters like this. By adding the word בני, Jacob hinted that Joseph himself had foreseen future developments in his dreams. Similarly, Jacob could not now act in defiance of G'd who had shown him in a vision that the younger son would outshine his older brother.