Bush's office issued a statement saying the 89-year-old former commander in chief and other members of his Secret Service detail shaved their heads to show solidarity with the 2-year-old boy, identified only as "Patrick" to protect his family's privacy, Fox News and The Washington Times reported.
The statement said Bush and his wife, who are staying at their summer home in Kennebunkport, Maine, made a donation to a fund set up for the family, www.patrickspals.org.
The security detail members have organized a motorcycle benefit run for Aug. 10 in Maine.
The Bushes lost their third child, Robin, to leukemia almost 60 years ago when she was 4.
In a statement on Patrick's website, the family thanked the Bushes for their "generosity and concern."
"Our little guy received some high-powered support this week when President George Bush 41 volunteered to shave his head to show his support for Patrick and Patrick's Pals," the family said.
"Members of his Secret Service detail have also been shaving their heads this week in solidarity."