I think he should donate it all to cancer research. By doing so he would not only be making an investment locally but globally and his investment could, potentially, have a direct impact on each and every one of us.
The Gates Foundation is dedicated medical and educational advances. Money goes to various illnesses that effect third-world countries.

From their official statement:
"We support efforts to prevent and treat diseases and conditions that meet three criteria: (1) they cause widespread illness and death in developing countries; (2) they represent the greatest inequities in health between developed and developing countries; and (3) they receive inadequate attention and funding. "

There is plenty of money already in Cancer research, their foundation is focused on supporting research into illnesses that have high fatality rates. Cancer isn't as wide spread in developing nations, you could almost blame it on today's technology and the slow roast we're putting our bodies through with all the broadcasted waves.