All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the deceased did not have any other heirs than those mentioned in the question, the mother gets one-sixth of the estate because the deceased has descendants entitled to inherit. Allaah, The Exalted, says (what means): {… And for one's parents, to each one of them is a sixth of his estate if he left children…}[Quran 4:11]
The remainder of the estate is to be given to the deceased’s son and daughter by virtue of Ta‘seeb (i.e. by virtue of having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed); the male gets double the share of the female.
Thus, the estate should be divided into 18 shares; the mother gets one-sixth of them (i.e. 3 shares), the son gets ten shares and the daughter gets 5 shares.
The rest of the heirs are fully excluded by the presence of the son.
It should be noted that the division of the inheritance is a serious and critical matter. It is not sufficient here to settle for seeking a Fatwa on the matter. Instead, you should refer the case to Sharee‘ah-court for investigation; perhaps other heirs entitled to inherit would be discovered in the process. Moreover, perhaps the deceased had made a bequest or was liable for debts or other rights of which the heirs are not aware. It is well-known that the repayment of debts and the liabilities should be given priority over the heirs’ right in the estate. Therefore, you should not divide the estate among the heirs without referring the case to Sharee‘ah-court, if any, in order to secure the interests of both the living and the dead.