My grandfather left a will which doesn't do justice to my father. Help

To challenge a will you must show that there is either something wrong with the will or that there was something wrong about the conditions under which the will was made. For example, you can challenge a will by claiming that:

the will does not meet formal requirements, such as the requirement for the signatures of two witnesses
the will maker did not have the legal capacity to make a will, or
the will maker made the will under suspect conditions, like fraud, mistake, or improper influence.
Flaw in Document Requirements
A formal will document must be signed by the will maker (with testamentary capacity, see below) and by two adults who will not receive anything under the will. For most formal wills, those are the only requirements to make a will document valid. In fact, valid will can be very simple, and most wills are lengthy only to set out its terms, not to make it valid.

In some states, if the will maker writes the entire will in his or her own handwriting, no witnesses are required. These are called “holographic” wills. However, they are usually not a good idea unless your circumstances keep you from being able to make a formal, witnessed will.

Lack of Capacity
A valid will requires that the will maker possessed "testamentary capacity," at the time the will was made. Sometimes this is known as “being of sound mind,” and it generally means that the will maker must know what he or she is doing by making a will. That is, he or she must have a clear understanding about what he or she owns and about what it means to leave it to someone else through a will. Capacity also requires that the will maker meet the age requirements set by the state. This age is usually 18, but some states allow younger people to make wills if they are married or in the military.

When challenging a will on the basis that the decedent did not understand the nature and extent of his or her property or who the property would go to without a will, you may be required to provide medical evidence showing mental impairment.

Undue Influence
A challenge of undue influence involves showing that the decedent did not make the will of free choice, but rather under the improper influence of another person. For example, if a lone sibling cares for an unwell parent, and the parent’s will gives that sibling a greater portion of the estate than other siblings, the other siblings may challenge the will saying that the caregiver sibling pressured the parent to change the will.

Fraud or Mistake
A will can also be challenged for fraud – for example if a challenger can show that pages have been inserted into the will after the decedent signed it or that the decedent's signature was forged.

Mistake is also grounds for a challenge. For example, if a mother’s will says that the majority of her estate should go to “John R. Smith” (the name of her nephew), when her son’s name is “John B. Smith”, her son could challenge the will on the basis that his mother simply made a typing error.