How binding is an original will that has been signed and witnessed?

I think the son's odds of success are very, very long here, even assuming that the son isn't the husband's child.  Assuming the attorney was competent, and the form and execution of the will are proper, there are most likely only two possible grounds to challenge the will's validity.  One of these is lack of capacity, and that's difficult because a person doesn't have to be fully competent to be able to make a will, they only have to have an understanding of their property and the people that they would naturally think about including in their will -- even if they try to stop thunderstorms by going outside and yelling at the sky (that's from a real case!).  The other one is undue influence, which means that the will wasn't "really" written by the deceased, but by someone else taking unfair advantage of a confidential relationship or dependency.  This is how a will leaving everything to the home health care aide gets thrown out -- but it is next to impossible, against a surviving spouse, unless there are some fairly extreme circumstances that can be shown with solid evidence.