Until your son is an adult it is usually a parent .....who is classed as next of kin so they would technically be entitled to your estate if you died
Sorry Namethecat but that is complete rubbish. Her son is her beneficiary whatever age he is and would therefore get her estate. It will be held in trust until 18 without a will. A will enables her to specify a different age for him to inherit, or even lump sums at different ages.
Op, you need to decide what age you want DS to inherit, who you want to be trustee(s), whether you want to specify for him to have money drip fed for driving lessons, car, uni etc, or just left to the discretion of the trustees.
If you think he will live with his father, and both are happy with that, then state that. Would you be happy for his dad to be a trustee of the money? You can use solicitors as trustees but it will cost!
You also can specify who inherits if you both die together, as others have said.
My DC have inherited at 18, from a cousin of mine, we have guided them to do a basic DIY will each, leaving the money to each other, and to cousins if both died together. But that is a very straightforward scenario compared to yours.
In your situation, using a professional for will writing is the best idea, as they can give suggestions as to what you need to consider. There is a MN poster who does them - I think it is marlow wills and she comes highly recommended by others.
If you dont make a will and your DS inherits at 18, would he cope? Would his father guide him to use the money wisely or would he fritter it? My niece also inherited as per my DC. She has since died herself, young and unexpectedly, and has seemingly frittered a lot of her inheritance, from what her parents can tell. Whereas DH and I are both quite money savvy, so are guiding our two to invest it and keep it for house purchases.