In general, "sound mind" only requires that:
1) He realize that he is making a last will and testament and what doing so means;
2) Comprehend who his family and friends are (the "natural objects of one's bounty");
3) Understand what he owns.
The best people to make that judgment are probably those treating him. Good witnesses and the written opinion of a medical doctor should be sufficient to prove that he has the mental capacity to make a will.