Hi Jane,
1. Generally, the testator (will maker) has absolute freedom in how they wish to distribute their assets after their death.
2. You cannot contest a will until the testator is deceased and the executor is applying for probate. This is because prior to death the will may be changed or altered and has no legal effect.
3. A potential beneficiary (spouse/partner, child of the deceased or any other person being maintained by the deceased immediately prior to his death) can contest a will after the testator's death. Spouse/partner generally should have some share in the assets (a life estate counts) and any other person must show that they were dependant on the deceased or maintained by him for some time prior to his death.
4. Alternatively, you can contest the will itself (i.e. argue that it is invalid). To do this, you will need to show that the testator lacked mental capacity to make the amendments/will at the time of execution. Therefore, you will need to show that the testator (1) did not know the significance of making a will; (2) did not fully comprehend the extent of his assets; and (iii) did not turn his mind to any potential beneficiary who may lay claim to his estate (i.e. children, grandchildren, spouse/partner).
Remember that these are his assets. Nobody has an interest or claim to them as of right. Any disposition under the will is purely a gift. Of course, for a spouse/partner, if you were to contest the will, the court will consider what you would likely have gotten if the death was a divorce/separation instead and in that case, what would have been reasonable under a financial division settlement. But for anyone else, it should be solely up to the will-maker's discretion.