what is the correct position of soon in these sentences Soon you will have forgotten him or You will soon have forgotten him?

"Soon, you will have forgotten him" and "You will soon have forgotten him" are both grammatically correct; however, while the first sounds better, neither sounds very natural. A better phrasing, if it fits the context, would be "You will soon forget him." Alternatively, you could keep the motherly tone by saying "Soon enough, you will have forgotten (all) about him," or a variation of the like.