Boy, 10, begins Kilimanjaro climb
Jack Harley-Walsh, of Ascot, will trek the 19,330ft (5,892m) peak in north east Tanzania, with his mother Sue.
The pair have already conquered Mount Meru to acclimatise for the climb and set off for Kilimanjaro on Saturday.
Money is being raising for the Thames Valley and Chiltern Air Ambulance Trust and Sebastian's Action Trust.
Jack said he was raising money for the air ambulance after visiting a fun day it held.
Sebastian's Action Trust was set up in memory of Sebastian Gates, from Binfield, near Wokingham, who died of cancer in 2003 aged nine.
It operates a holiday home where sick children can visit with their families.
Grandmother Josie Walsh told the BBC that when they reached the top of Mount Meru the youngster and his mother raised a flag wearing their charity t-shirts.
"They said they were going to take it slow on the first day of Kilimanjaro just to give Jack every chance to get to the top, the guides have been absolutely fantastic.
"He just hopes that his chickens and kittens are alright back home.
"We are encouraging people to sponsor him so we can raise as much money as we can."
The group set off for Kilimanjaro on Saturday after a day's rest at base camp and are due to return on Tuesday.