Wills, probate, letters of administration, powers of attorney
Anyone can contest a will, but to successfully contest it valid grounds are needed.
For an adult they need to show that they have been dependent on you during your lifetime and they have a need.
The fact that you have left what might have been his share to his children would have no bearing in any defense of a challenge to the will.
I believe your will should state why your son has been excluded to ensure any actions by him after your death are suitably countered.
The primary deciding factor of a successful challenge to your will is your sons need versus the other beneficiaries.
What your relationship is/was has little bearing. Although it is wise to clearly state why you have disinherited someone, it makes very little difference as the decision is about what a wise and just testator should have done.
So if you son could use the money he is in with a chance.
The state you reside in when you die is a major factor. NSW being the easiest to win. With Vic a close second.
Thankyou Citizen Joe and Goingbonkers for your advice. Son is not short of funds and lives afluently so maybe wouldn't win if contested. He is 45 so not dependant on us. We will add a letter with the will explaining everything. He will still get a rude awakening at the time which we hope is not for many years to come. Thanks again.