Is it permissible to donate a sum from the money of a deceased as an ongoing charity to benefit his soul without harming his son's interests?

The nisab [minimum sum upon which zakat is due] for zakat on wealth is the equivalent of 85 grams of 21 carat gold and zakat is 2.5%of the original sum. The same percentage is due on the interest if it has been in the owner’s possession for one lunar year. Otherwise, no zakat is due on it.

After death, a deceased ceases to own any money except for the expenses he needs for his funeral and burial, any bequests that are to be deducted from the money he leaves behind, and the money needed to settle his debts. Anything in surplus of this belongs to his heirs.

The ruling
It is impermissible to give an ongoing charity from the estate of the deceased because the heirs include a minor who is not yet legally accountable. This is because it is impermissible to dispose of his property except if it is in his sole interest which giving out in charity for the sake of his father is not. However, each of the heirs — you or any other — may give out an ongoing charity for the benefit of the soul of the deceased, either collectively or individually, since you are legally accountable and will give it out of your own free will.
And God the Almighty knows best.